Patch merged into coreboot/master: cbmem: Add comment for cbmem_top returning NULL if backing store is not ready

the following patch was just integrated into master:
commit 6f009dc485e8bfb0a9f316c1e831938cf46b0c3a
Author: Furquan Shaikh 
Date:   Mon Mar 28 13:23:13 2016 -0700

cbmem: Add comment for cbmem_top returning NULL if backing store is not ready

Board or chipset needs to ensure that cbmem backing store is ready
    when returning the cbmem top address. cbmem infrastructure has no
    support for checking the validity of the backing store/address.

E.g.: If romstage handles cbmem coming online, chipset or board need
    to ensure that call to cbmem_top in romstage returns NULL if the
    backing store is not yet initialized.

Add a comment to ensure that developers know this requirement while
    implementing cbmem_top for future chipsets/boards.

Change-Id: I0086b8e528f65190b764a84365cf9bf970b69c3f
    Signed-off-by: Furquan Shaikh 
    Reviewed-on: https://review.coreboot.org/14181
    Tested-by: build bot (Jenkins)
    Reviewed-by: Nico Huber 
    Reviewed-by: Aaron Durbin 
    Reviewed-by: Paul Menzel

See https://review.coreboot.org/14181 for details.

-gerrit