ar9170-fw: Boostrap our own toolchain

To avoid semi-open binary toolchains: This patch adds a toolchain subdir and Makefile to download, build and install our own gnu sh2 toolchain into that directory.

---

 toolchain/Makefile |   54 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 54 insertions(+), 0 deletions(-)
 create mode 100644 toolchain/Makefile

index 6dd924f..d2fa705 100644
@@ -5,3 +5,6 @@
 fw.map
 *~
 *.dis
+toolchain/src
+toolchain/build
+toolchain/inst