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>
diff --git a/src/lib/timestamp.c b/src/lib/timestamp.c
index d8b93f9..2f2a5d8 100644
--- a/src/lib/timestamp.c
+++ b/src/lib/timestamp.c
@@ -27,6 +27,7 @@
 
 #define MAX_TIMESTAMPS 84
 
+/* When changing this number, adjust TIMESTAMP() size ASSERT() in memlayout.h */
 #define MAX_BSS_TIMESTAMP_CACHE 16
 
 struct __attribute__((__packed__)) timestamp_cache {