crosfw: Select board by target name only

At present we search the boards.cfg file to find the architecture for a
board to be built. But the supplied board can match any field in the board
line. In the case of Raspberry Pi ('rpi') this can match an aarch64 target
with rpi_3, or an arm target with rpi. Fix it to distinguish these
correctly.

Signed-off-by: Simon Glass <sjg@chromium.org>
BUG=chromium:629855
TEST=crosfw -b rpi -w; see that it builds with the correct (arm) toolchain

Change-Id: Idf1a3f475851c38d76fd0b9b55a0c30d8867be24
Reviewed-on: https://chromium-review.googlesource.com/411472
Commit-Ready: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed