vboot2: add verstage

This reverts the revert commit 5780d6f387
and fixes the build issue that cuased it to be reverted.

Verstage will host vboot2 for firmware verification.
It's a stage in the sense that it has its own set of toolchains,
compiler flags,
and includes. This allows us to easily add object files as needed. But
it's directly linked to bootblock. This allows us to avoid code
duplication for stage loading and jumping (e.g. cbfs driver) for the
boards
where bootblock has to run in a different architecture (e.g. Tegra124).
To avoid name space conflict, verstage symbols are prefixed with
verstage_.

TEST=Built with VBOOT2_VERIFY_FIRMWARE on/off. Booted Nyan Blaze.
BUG=None
BRANCH=none

Original-Signed-off-by: Daisuke Nojiri <dnojiri@chromium.org>
Original-Change-Id: Iad57741157ec70426c676e46c5855e6797ac1dac
Original-Reviewed-on: https://chromium-review.googlesource.com/204376
Original-Reviewed-by: Randall Spangler <rspangler@chromium.org>

(cherry picked from commit 27940f8916)
Signed-off-by: Marc Jones <marc.jones@se-eng.com>
Change-Id: I2a83b87c29d98d97ae316091cf3ed7b024e21daf
Reviewed-on: http://review.coreboot.org/8224
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/cpu/allwinner/a10/Kconfig

@@ -7,6 +7,7 @@ if CPU_ALLWINNER_A10
 config CPU_SPECIFIC_OPTIONS
 	def_bool y
 	select ARCH_BOOTBLOCK_ARMV7
+	select ARCH_VERSTAGE_ARMV7
 	select ARCH_ROMSTAGE_ARMV7
 	select ARCH_RAMSTAGE_ARMV7
 	select HAVE_MONOTONIC_TIMER