[S] Change in coreboot[main]: x86: Add ramstage CBFS cache support

[Jérémy Compostella (Code Review)]

Attention is currently required from: Arthur Heymans, Julius Werner, Karthik Ramasubramanian, Maximilian Brune.

Hello build bot (Jenkins),

I'd like you to reexamine a change. Please visit

https://review.coreboot.org/c/coreboot/+/77885?usp=email

to look at the new patch set (#24).

Change subject: x86: Add ramstage CBFS cache support
......................................................................

x86: Add ramstage CBFS cache support

This commit introduces the x86 `RAMSTAGE_CBFS_CACHE_SIZE' Kconfig to
set the ramstage CBFS cache size.  A cache size of zero disables the
CBFS cache feature.  The default size is 16 KB which seems a
reasonable minimal value large enough to satisfy basic needs such as
the decompression of a small configuration file.  This setting can be
adjusted depending on the platform needs and capabilities.

To support S3 suspend/resume use-case, the CBFS cache memory cannot be
released to the Operating System. There are two options to meet this
requirement:

1. Define a static CBFS cache buffer (located in the .bss section)
2. Create a new CBMEM entry

Option #2 seems more powerful but considering that:

1. The CBFS cache is actually not a cache but just a scratch pad
   designed to be isolated between stages
2. postcar is a very short stage not really needing CBFS cache
3. The static initialization of the `cbfs_cache' global
   variable (cf. src/lib/cbfs.c) offer a simple and robust design

=> It is simpler to use a static buffer and limit the support to
ramstage.

Since some AMD SoCs (cf. `SOC_AMD_COMMON_BLOCK_NONCAR' Kconfig)
defines a `_cbfs_cache' region, an extra `POSTRAM_CBFS_CACHE_IN_BSS'
Kconfig must be set to enable the use of a static buffer as the CBFS
cache scratchpad.

TEST=Decompression of vbt.bin in ramstage on rex using cbfs_map()

Change-Id: I7fbb1b51cda9f84842992e365b16c5ced1010b89
Signed-off-by: Jeremy Compostella 
---
M src/arch/x86/Kconfig
M src/lib/cbfs.c
2 files changed, 23 insertions(+), 0 deletions(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/85/77885/24