coreboot: introduce CONFIG_RELOCATABLE_RAMSTAGE

This patch adds an option to build the ramstage as a reloctable binary.
It uses the rmodule library for the relocation. The main changes
consist of the following:

1. The ramstage is loaded just under the cmbem space.
2. Payloads cannot be loaded over where ramstage is loaded. If a payload
   is attempted to load where the relocatable ramstage resides the load
   is aborted.
3. The memory occupied by the ramstage is reserved from the OS's usage
   using the romstage_handoff structure stored in cbmem. This region is
   communicated to ramstage by an CBMEM_ID_ROMSTAGE_INFO entry in cbmem.
4. There is no need to reserve cbmem space for the OS controlled memory for
   the resume path because the ramsage region has been reserved in #3.
5. Since no memory needs to be preserved in the wake path, the loading
   and begin of execution of a elf payload is straight forward.

Change-Id: Ia66cf1be65c29fa25ca7bd9ea6c8f11d7eee05f5
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/2792
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@google.com>

src/lib/Makefile.inc

@@ -110,6 +110,7 @@ $(obj)/lib/uart8250.smm.o : $(OPTION_TABLE_H)
 
 ifeq ($(CONFIG_RELOCATABLE_MODULES),y)
 ramstage-y += rmodule.c
+romstage-$(CONFIG_RELOCATABLE_RAMSTAGE) += rmodule.c
 
 RMODULE_LDSCRIPT := $(src)/lib/rmodule.ld
 RMODULE_LDFLAGS  := -nostartfiles -shared -z defs -nostdlib -Bsymbolic -T $(RMODULE_LDSCRIPT)