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/soc/intel/baytrail/northcluster.c

@@ -51,7 +51,7 @@
 
 uint32_t nc_read_top_of_low_memory(void)
 {
-	MAYBE_STATIC_BSS uint32_t tolm = 0;
+	static uint32_t tolm;
 
 	if (tolm)
 		return tolm;

src/soc/intel/braswell/northcluster.c

@@ -56,7 +56,7 @@
 
 uint32_t nc_read_top_of_low_memory(void)
 {
-	MAYBE_STATIC_BSS uint32_t tolm = 0;
+	static uint32_t tolm;
 
 	if (tolm)
 		return tolm;