Remove MAYBE_STATIC_BSS and ENV_STAGE_HAS_BSS_SECTION

After removal of CAR_MIGRATION there are no more reasons
to carry around ENV_STAGE_HAS_BSS_SECTION=n case.

Replace 'MAYBE_STATIC_BSS' with 'static' and remove explicit
zero-initializers.

Change-Id: I14dd9f52da5b06f0116bd97496cf794e5e71bc37
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40535
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

toolchain.inc

@@ -55,7 +55,7 @@ CFLAGS_ppc64	+=
 # stack use, we use 1.5K as heuristic, assuming that we typically have lots
 # of tiny stack frames and the odd large one.
 #
-# Store larger buffers in BSS, use MAYBE_STATIC_BSS to share data in cache-as-ram
+# Store larger buffers in BSS, use static to share data in cache-as-ram
 # on x86.
 # Since GCCs detection of dynamic array bounds unfortunately seems to be
 # very basic, you'll sometimes have to use a static upper bound for the