add documentation for configure --enable-linux-build option.

Index: README.INSTALL
===================================================================
@@ -18,13 +18,20 @@
 target kernel, or as loadable modules as with earlier versions.
 therefore, the usual Linux kernel configuration process will be
 normally used to define the various settings for the Xenomai kernel
-components.
+components. Sections 1.1 and 1.2 document the installation process 
+of this kernel space support.
 
 The src/ sub-tree contains the various user-space libraries and
 commands provided by the Xenomai framework. This tree can be built
 separately from the kernel support, even if the latter is absent from
-the build system.
+the build system. Section 1.3 subsections document the installation
+process of this user-space support.
 
+Finally, Xenomai build system also allow building a Xenomai patched 
+kernel as part of the user space support compilation process. Section 
+1.4 document this possibility.
+
+
 1. Installation steps
 ---------------------
 

@@ -189,6 +196,51 @@
 
 See sections 2.2, 2.3 and 2.4 for examples.
 
+1.4 Building the Linux kernel in the user-space support build tree
+------------------------------------------------------------------
+
+Having two build trees, one for the user space support and one for the
+kernel space support may be error prone, because some modifications
+require rebuilding both trees and one easily forget to rebuild the
+kernel space support. In order to cope with this issue, Xenomai allow 
+to build the kernel space support in a subdirectory of the user space 
+support build tree.
+
+In order to enable this functionality, pass to the user space support 
+configure script the option:
+--enable-linux-build=$linux_tree 
+where $linux_tree is the path to a Linux kernel source tree to be 
+copied.
+
+The configure script then create a Linux kernel source tree in the 
+"linux" sub-directory of the build directory, where all files are 
+symbolic links to the files of the $linux_tree source tree. The 
+prepare-kernel script is then called with the correct --linux, --arch 
+and --adeos parameters.
+
+Then, "make xconfig", "make menuconfig", "make oldconfig" allow to 
+configure the Linux kernel; each time "make" is run at the build tree 
+top, the Linux kernel and modules are rebuilt if needed, and when
+"make install" is run, the Linux kernel and modules are installed. If
+a DESTDIR variable is passed with "make install", the Linux kernel and 
+modules are installed under the directory $DESTDIR. Other targets of 
+the Linux build system are accessible by prefixing them with linux/, 
+for example running make linux/help run "make help" in the Linux tree.
+
+See section 2.5 for an example.
+
+When re-running configure in the build tree with a different parameter
+passed to --enable-linux-build, or if the Adeos patch version changed
+in the source tree (for example, as the result of running svn update),
+configure will display the following message:
+*** Warning: built version of linux and requested versions are
+*** different. If you want to build the requested version of
+*** linux, remove the file: linux/.xenomai-prepared.
+
+Removing the linux/.xenomai-prepared file and running make again 
+should be enough to compile the new Linux kernel version with the
+same configuration.
+
 2. Typical installation procedures
 ----------------------------------
 

@@ -279,3 +331,17 @@
 You may also want to have a look at this hands-on description about
 configuring and building a Xenomai system for the Blackfin
 architecture: http://docs.blackfin.uclinux.org/doku.php?id=adeos
+
+2.5 Building for x86 with the --enable-linux-build option
+---------------------------------------------------------
+
+Build the Linux kernel as part of the user space support build process:
+
+$ mkdir $build_root && cd $build_root
+$ $xenomai_root/configure --enable-linux-build=$linux_tree --enable-x86-sep
+$ make xconfig/gconfig/menuconfig # select the kernel and Xenomai options
+$ make install
+
+When "make install" is finished, you should find the patched kernel installed
+in /boot, its module under /lib/modules, and Xenomai user-space libraries
+and tools under /usr/xenomai.