nb/intel/nehalem: Move to C_ENVIRONMENT_BOOTBLOCK

A few notable changes:
- Microcode init is done in assembly during the CAR init.
- The DCACHE_BSP_STACK_SIZE is set to 0x2000, which is the same size
  against which the romstage stack guards protected.
- The romstage mainboard_lpc_init() hook is removed in favor of the
  existing bootblock_mainboard_early_init().

Change-Id: Iccd7ceaa35db49e170bfb901bbff1c1a11223c63
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35951
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/southbridge/intel/ibexpeak/Kconfig

@@ -51,10 +51,6 @@ config DRAM_RESET_GATE_GPIO
 	int
 	default 60
 
-config BOOTBLOCK_SOUTHBRIDGE_INIT
-	string
-	default "southbridge/intel/ibexpeak/bootblock.c"
-
 config SERIRQ_CONTINUOUS_MODE
 	bool
 	default n