Exit building if romstage.bin is larger than size of XIP

Exit building if romstage.bin is larger than size of XIP

When the romstage.bin becomes bigger than the size of XIP, the
cbfstool can not allocate the romstage in the CBFS. But it doesn't
report an error. It will take quite a while to find out the root
cause.

Change-Id: I5be2a46a8b57934f14c5a0d4596f3bec4251e0aa
Signed-off-by: Zheng Bao <zheng.bao@amd.com>
Signed-off-by: zbao <fishbaozi@gmail.com>
Reviewed-on: http://review.coreboot.org/650
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marcj303@gmail.com>