abuild: change board identifier to a variant of CONFIG_BOARD_*

abuild: change board identifier to a variant of CONFIG_BOARD_*

Since we now have multiple boards in a single mainboard directory (eg
google/veyron), we need some other identifier from which to create
output directories and filenames in abuild than the directory name.
Use the wildcard part of CONFIG_BOARD_* instead.

This changes the semantics of payload.sh handling: it's passed the
single new identifier instead of two arguments "vendor" and "board" that
constitute the mainboard directory's path.

Change-Id: I0dc59c6a1ad1ee51d393fa06b98944a6da342cdf
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: http://review.coreboot.org/12277
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>