payloads/external/GRUB2: prevent rebuild without actual changes

When the coreboot config changes, grub2 gets rebuilt, too. This is
caused by Make detecting a change in the dependency on the config file.

There are two cases where grub2 needs to be rebuilt (partly):

  1. Changed list of included modules (CONFIG_GRUB2_EXTRA_MODULES)
     In this case only the resulting binary has to be repackaged with
     (or without) the modules changed. This does not require a full
     rebuild and is intellegently handled by Make / the build system.

  2. Changed grub revision or branch in coreboot config
     In this case grub has to be recompiled from the new sources.
     Fortunately all this is - again - magically handled by the build
     system (automake and Make, to be precise) and there is no need to
     influence that process from coreboot, like removing the build
     directory for example.

This makes it possible to reduce build times after coreboot config
changes by adjusting the payload's Make targets and their dependencies
and letting the build system any required (partial or full) rebuilds.

Further, this commit removes the redundant dependeny on `build/config.h`
and drops the superfluous `CONFIG_DEP` make parameter.

Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Change-Id: Ibf5a316d0c9761426ec2ee306d78cd56d4bf19b5
---
M payloads/external/GRUB2/Makefile
1 file changed, 2 insertions(+), 4 deletions(-)