setup_board: support generic target make.conf

Since the contents of target make.conf files are often the same, support
falling back to a generic one if there is no arch-specific one.

BUG=chromium:307180
TEST=building for ppc32 works

Change-Id: I8c4d933ab90d9332e15a0f6b0cc28d61a5ffe6f3
Reviewed-on: https://chromium-review.googlesource.com/173676
Reviewed-by: David James <davidjames@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed