timestamp: Remove HAS_PRECBMEM_TIMESTAMP_REGION Kconfig

This patch generalizes the approach previously used for ARM32
TTB_SUBTABLES to "auto-detect" whether a certain region was defined in
memlayout.ld. This allows us to get rid of the explicit Kconfig for the
TIMESTAMP region, reducing configuration redundancy and avoiding
confusion when setting up future boards.

(Removing armv4/bootblock_simple.c because it references this Kconfig
and it is a dead file that I just forgot to remove in CL:12076.)

BRANCH=None
BUG=None
TEST=Booted Oak and confirmed that all pre-RAM timestamps are still
there. Built Nyan and Falco.

Change-Id: I557a4b263018511d17baa4177963130a97ea310a
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/13652
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/include/symbols.h

@@ -84,4 +84,11 @@ extern u8 _framebuffer[];
 extern u8 _eframebuffer[];
 #define _framebuffer_size (_eframebuffer - _framebuffer)
 
+/* Put this into a .c file accessing a linker script region to mark that region
+ * as "optional". If it is defined in memlayout.ld (or anywhere else), the
+ * values from that definition will be used. If not, start, end and size will
+ * all evaluate to 0. (We can't explicitly assign the symbols to 0 in the
+ * assembly due to https://sourceware.org/bugzilla/show_bug.cgi?id=1038.) */
+#define DECLARE_OPTIONAL_REGION(name) asm (".weak _" #name ", _e" #name )
+
 #endif /* __SYMBOLS_H */