arm/arm64: Generalize bootblock C entry point

When we first added ARM support to coreboot, it was clear that the
bootblock would need to do vastly different tasks than on x86, so we
moved its main logic under arch/. Now that we have several more
architectures, it turns out (as with so many things lately) that x86 is
really the odd one out, and all the others are trying to do pretty much
the same thing. This has already caused maintenance issues as the ARM32
bootblock developed and less-mature architectures were left behind with
old cruft.

This patch tries to address that problem by centralizing that logic
under lib/ for use by all architectures/SoCs that don't explicitly
opt-out (with the slightly adapted existing BOOTBLOCK_CUSTOM option).
This works great out of the box for ARM32 and ARM64. It could probably
be easily applied to MIPS and RISCV as well, but I don't have any of
those boards to test so I'll mark them as BOOTBLOCK_CUSTOM for now and
leave that for later cleanup.

BRANCH=None
BUG=None
TEST=Built Jerry and Falco, booted Oak.

Change-Id: Ibbf727ad93651e388aef20e76f03f5567f9860cb
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: http://review.coreboot.org/12076
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins)
diff --git a/src/soc/marvell/bg4cd/Kconfig b/src/soc/marvell/bg4cd/Kconfig
index 255ce57..2e5b15c 100644
--- a/src/soc/marvell/bg4cd/Kconfig
+++ b/src/soc/marvell/bg4cd/Kconfig
@@ -17,10 +17,10 @@
 	bool
 	default n
 	select ARCH_BOOTBLOCK_ARMV7_M
+	select BOOTBLOCK_CUSTOM if VBOOT_VERIFY_FIRMWARE
 	select ARCH_RAMSTAGE_ARMV7
 	select ARCH_ROMSTAGE_ARMV7
 	select ARCH_VERSTAGE_ARMV7_M
-	select ARM_BOOTBLOCK_CUSTOM if VBOOT_VERIFY_FIRMWARE
 	select BOOTBLOCK_CONSOLE
 	select GENERIC_UDELAY
 	select HAVE_MONOTONIC_TIMER
@@ -31,8 +31,4 @@
 config CHROMEOS
 	select VBOOT_STARTS_IN_BOOTBLOCK
 
-config BOOTBLOCK_CPU_INIT
-	string
-	default "soc/marvell/bg4cd/bootblock.c"
-
 endif