memlayout: Ensure TIMESTAMP() region is big enough to avoid BUG()

The timestamp code asserts that the _timestamp region (allocated in
memlayout for pre-RAM stages) is large enough for the assumptions it
makes. This is good, except that we often initialize timestamps
extremely early in the bootblock, even before console output. Debugging
a BUG() that hits before console_init() is no fun.

This patch adds a link-time assertion for the size of the _timestamp
region in memlayout to prevent people from accidentally running into
this issue.

Change-Id: Ibe4301fb89c47fde28e883fd11647d6b62a66fb0
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/16270
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: Raptor Engineering Automated Test Stand <noreply@raptorengineeringinc.com>

src/include/memlayout.h

@@ -75,7 +75,8 @@
 #define DRAM_START(addr) SYMBOL(dram, addr)
 
 #define TIMESTAMP(addr, size) \
-	REGION(timestamp, addr, size, 8)
+	REGION(timestamp, addr, size, 8) \
+	_ = ASSERT(size >= 212, "Timestamp region must fit timestamp_cache!");
 
 #define PRERAM_CBMEM_CONSOLE(addr, size) \
 	REGION(preram_cbmem_console, addr, size, 4)