Clean up architecture-specific Kconfigs

It's an unfortunate side effect of our different-archs-per-stage
mechanism that all src/arch/*/Kconfig files are always parsed with no
if blocks to exclude them if they're not relevant. This makes it very
easy to accidentally rely on a Kconfig default set by a totally
different and not applying architecture.

This patch moves a few Kconfigs from ARM and X86 that leaked out like
this into a common Kconfig file for clarity. It also gives ARM64 its
own BOOTBLOCK_CUSTOM mechanism so that it doesn't leech off the ARM one
(currently not used by any board).

In the future, we should maybe prefix all options in the arch/*/Kconfig
files with the architecture name (such as X86_BOOTBLOCK_NORMAL and
ARM_LPAE are already doing), to make it more apparent when they are used
in the wrong place.

BUG=None
TEST=None (tested together with dependent changes)

Change-Id: I3e8bb3dfbb2c4edada621ce16d130bd7387d4eb8
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: 5528aa9252cdf711af3c160da387c6a7bebe9e76
Original-Change-Id: Ieb2d79bae6c6800be0f93ca3489b658008b1dfae
Original-Signed-off-by: Julius Werner <jwerner@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/219171
Original-Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/9235
Reviewed-by: Aaron Durbin <adurbin@google.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/arch/x86/Kconfig

@@ -49,10 +49,6 @@ config RAMBASE
 	hex
 	default 0x100000
 
-config STACK_SIZE
-	hex
-	default 0x1000
-
 # This is something you almost certainly don't want to mess with.
 # How many SIPIs do we send when starting up APs and cores?
 # The answer in 2000 or so was '2'. Nowadays, on many systems,