soc/amd/common: Allow AGESA file split for pre- and post-memory

soc/amd/common: Allow AGESA file split for pre- and post-memory

By splitting the binary files for platform initialization, the
post-memory code can be modified to stop executing in place (--xip).

This change creates two separate sections in CBFS for AGESA and loads
the appropriate file at the correct stage.

BUG=b:68141063
TEST=Booted kahlee with split agesa enabled.

Change-Id: I2fa423df164037bc3738476fd2a34522df279e34
Signed-off-by: Justin TerAvest <teravest@chromium.org>
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/22974
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>