Build timestamp_autogenerated.h without config

Tools such as mkimage include version information but are
config-agnostic; build timestamp_autogenerated.h even when config.mk
isn't generated to fix "make tools" build failure:
gcc [...] -o mkimage.o mkimage.c -c
In file included from include/version.h:27:0, from mkimage.c:26:
include/timestamp.h:27:37: fatal error: timestamp_autogenerated.h: No
such file or directory

Cc: patches@linaro.org
Signed-off-by: Loïc Minier <loic.minier@linaro.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>