libpayload: Add ARM defconfig

make junit.xml tries to build it, but fails
(ARM port doesn't seem to be ready?)
Useful test case to demonstrate a failing
libpayload build.

Change-Id: Iba4fe551b48f631e6a3bd90eb07930fc70761332
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/4552
Tested-by: build bot (Jenkins)
Reviewed-by: Isaac Christensen <isaac.christensen@se-eng.com>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>