bootblock: Link timestamp.c only with EARLY_CBMEM_INIT

Commit dbeedbef (arch/x86/bootblock: Link in object files selected with
bootblock-y) breaks building of x86 boards with
`CONFIG_EARLY_CBMEM_INIT` *not* selected but CBMEM time stamp collection
enabled.

Aaron Durbin explained as below [1] and provided this patch to fix it.

> That change actually processes bootblock-objs where before it never did
> such a thing. I'm sure this isn’t the only issue lurking. bootblock on
> x86 implied romcc and thus all the bootblock-y += rules that other
> architectures use worked, but now all the implied assumptions are no
> longer true on x86.
>
> timestamp stuff on x86 !CONFIG_EARLY_CBMEM_INIT is the issue you're
> seeing. In order to compile timestamp.c for bootblock under these
> conditions will mean there needs to be some more Makefile guarding.

[1] http://review.coreboot.org/11864

Change-Id: I3441b9fcdbbc8bbe82b9f2075e60668a846ecf09
Fix-by: Aaron Durbin <adurbin@chromium.org>
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/11875
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Tested-by: build bot (Jenkins)

src/lib/Makefile.inc

@@ -28,7 +28,10 @@ bootblock-$(CONFIG_GENERIC_GPIO_LIB) += gpio.c
 bootblock-y += libgcc.c
 bootblock-$(CONFIG_GENERIC_UDELAY) += timer.c
 
+ifeq ($(CONFIG_EARLY_CBMEM_INIT),y)
 bootblock-$(CONFIG_COLLECT_TIMESTAMPS) += timestamp.c
+endif
+
 bootblock-$(CONFIG_CONSOLE_CBMEM) += cbmem_console.c
 bootblock-$(CONFIG_I2C_TPM) += delay.c
 bootblock-y += memchr.c