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>

src/lib/imd_cbmem.c

@@ -19,7 +19,7 @@ static struct imd imd;
 void *cbmem_top(void)
 {
 	if (ENV_ROMSTAGE) {
-		MAYBE_STATIC_BSS void *top = NULL;
+		static void *top;
 		if (top)
 			return top;
 		top = cbmem_top_chipset();