[Buildroot] [git commit master 1/1] binutils, gcc: use correct --prefix

[Thomas Petazzoni]

commit: http://git.buildroot.net/buildroot/commit/?id=2ae84ac85ff72a861e8c276ba4e650e1e9ba7982
branch: http://git.buildroot.net/buildroot/commit/?id=refs/heads/master

The cross binutils and cross gcc are actually going to be executed
from $(STAGING_DIR)/usr, so the correct prefix is $(STAGING_DIR)/usr
and not /usr.

This also fixes what is known as the "AVR32 toolchain build failure",
which was due to the fact that the prefix directory wasn't writable
(since it was /usr).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni at free-electrons.com>

Patch is too large, so refusing to show it