arch/x86: Implement minimal bootblock for C_ENVIRONMENT_BOTOBLOCK

Some newer x86 systems can boot from non-memory-mapped boot media
(e.g. EMMC). The bootblock may be backed by small amounts of SRAM, or
other memory, similar to how most ARM chipsets work. In such cases, we
may not have enough code space for romstage very early on. This means
that CAR setup and early boot media (e.g. SPI, EMMC) drivers need to
be implemented within the limited amount memory of storage available.
Since the reset vector has to be contained in this early code memory,
the bootblock is the best place to implement loading of other stages.

Implement a bootblock which does the minimal initialization, up to,
and including switch to protected mode. This then transfers control
to platform-specific code. No stack is needed, and control is
transferred via a "jmp" such that no stack operations are involved.

Change-Id: I009b42b9a707cf11a74493bd4d8c189dc09b8ace
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-on: https://review.coreboot.org/13485
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/arch/x86/Makefile.inc

@@ -112,6 +112,7 @@ $(call src-to-obj,bootblock,$(dir)/id.S): $(obj)/build.h
 
 ifeq ($(CONFIG_C_ENVIRONMENT_BOOTBLOCK),y)
 
+bootblock-y += bootblock_crt0.S
 bootblock-y += memlayout.ld
 
 ifeq ($(CONFIG_ARCH_BOOTBLOCK_X86_32),y)