Allow caller to specify build output directory

This changes the Makefile to set the BUILD directory only if the caller
does not specify it.

This allows U-Boot to build vboot reference and put the resulting binaries
into its own output directory.

BUG=chromium-os:16808
TEST=emerge vboot_reference-firmware for tegra2-seaboard, x86-mario

Change-Id: I6b0bba47e397f86a130e67c4e18fbcf5fffdfd68
Reviewed-on: https://gerrit.chromium.org/gerrit/11638
Reviewed-by: Bill Richardson <wfrichar@chromium.org>
Commit-Ready: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
1 file changed