core: allow for custom, local help; rearrange package-specific help (branch yem/help)

Hello All!

This series is an alternative proposal to support displaying custom help, most probably from a br2-external tree (or from a local.mk, but that'd be pretty insane).

The basic idea is to offer the user help entries for local customisations, but without mixing them with our own help entries.

I believe we want to keep them separate, as inter-mixing the two could lead users to believe those addtional help entries are "native" in buildroot, when they are not.

The same way we expose all of br2-external config options under a sub-menu labelled "User-provided options" (although this is mostly a technical limitation/artifact), we do similarly for the custom help, albeit with a slight shorter moniker, 'help-custom'.

The first two patches implement this new make target, the first by adding it as a purely separate and new target, the scond by making it appended after out internal help.

Third patch adds a new helper make function, that can be used to pretty-print the package-specific help. This macro is added to to the manual, as an *example* of what the custom help could use.

The macro is then used to implement per-package specific help (which is currently only about kconfig-related changes, but can not easily be done in the kconfig-package infra) into the packages themselves, to clean up the main help code (the entries are still displayed there, they are just defined in each concerned package).

Thanks to J?r?me for his initial proposal, and Arnout for before-inception suggestions:

    http://lists.busybox.net/pipermail/buildroot/2016-March/155415.html

Additional notes:

- only the first two patches are what is required to get custom help. The followup patches only address the other part of J?r?me's patch, which is about moving patckage-specifi help to the concerned packages;

- the new custom help is not intermixed with our internal help; rather

    it is appended to it;

- indeed, this macro does not allow packages to expose arbitrary help. It does not however prevent packages from documenting arbitrary rules. If a package wants to provide arbitrary help, then it would probably do in its .mk file something like:
LIBFOO_HELP = "libfoo-help : print help for libfoo" libfoo-help:
@cat my-biggish-help-file

Changes v1 -> v2:
- rename the custom help make target
- make it explicit that it is *absolutely* free-form
- better document it in the manual
- print custom help after our internal help
- properly pretty-print on multi lines

(Sorry, I haven't carried Arnout's reviews so far...)

Regards,
Yann E. MORIN.

The following changes since commit 0b7b84310c3781c346eea4c114c8cb4e97bbd8ea:

olimex_imx233_olinuxino_defconfig: genimage support (2016-03-11 13:14:54 +0100)

are available in the git repository at:

git://git.busybox.net/~ymorin/git/buildroot yem/help

for you to fetch changes up to b508f49279beea5e3f1b823408aa12ad7d9a35f9:

boot/barebox: use the generic help rules (2016-03-11 18:20:45 +0100)

----------------------------------------------------------------
Yann E. MORIN (9):
core: add the possibility to provide help for custom rules
core: also display the custom help with our main help core/pkg-utils: add a macro to pretty-print a help entry docs/manual: print-help can be used in help-custom core/pkg-generic: add help about package-specific rules package/busybox: use the generic help rules
linux: add the generic help rules package/uclibc: use the generic help rules boot/barebox: use the generic help rules

 Makefile                                | 27 ++++++++++-----------------
 boot/barebox/barebox.mk                 |  4 ++++
 docs/manual/adding-packages-generic.txt |  6 ++++++
 docs/manual/customize-outside-br.txt    | 26 ++++++++++++++++++++++++++
 linux/linux.mk                          |  6 ++++++
 package/busybox/busybox.mk              |  2 ++
 package/pkg-generic.mk                  |  2 ++
 package/pkg-utils.mk                    | 22 ++++++++++++++++++++++
 package/uclibc/uclibc.mk                |  2 ++
 9 files changed, 80 insertions(+), 17 deletions(-)

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