Makefile: Don't run genbuild_h if not doing a build

Makefile: Don't run genbuild_h if not doing a build

genbuild_h was being run on every make invocation - clean, distclean,
etc. to get the source date epoch value.  This value isn't used unless
a build is being done, so don't run it on non-compile make invocations.

Signed-off-by: Martin Roth <martin@coreboot.org>
Change-Id: I2afc0affc17116e0db849ea968474bc19dbb0ae1
Reviewed-on: https://review.coreboot.org/c/coreboot/+/53997
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>