make GRUB multi-tree and re-add xhci patches

Re-add xHCI only on haswell and broadwell machines, where
they are needed. Otherwise, keep the same GRUB code.

The xHCI patches were removed because they caused issues
on Sandybridge-based Dell Latitude laptops. See:
https://codeberg.org/libreboot/lbmk/issues/216

The issue was not reported elsewhere, including on the
Haswell/Broadwell hardware where they are needed, but the
build system could only build one version of GRUB.

The older machines do not need xHCI patches, because they
either do not have xHCI patches, or work (in GRUB) because
they're in EHCI mode when running the payload.

So, the problem is that we need the xHCI patches for GRUB
on Haswell/Broadwell hardware, but the patches break
Sandybridge hardware, and we only had the one build of GRUB.
To mitigate this problem, the build system now supports
building multiple revisions of GRUB, with different patches,
and each given coreboot target can say which GRUB tree to use
by setting this in target.cfg:

grubtree="xhci"

In the above example, the "xhci" tree would be used. Some
generic GRUB config has been moved to config/data/grub/
and config/grub/ now looks like config/coreboot/ - also,
the grub.cfg file (named "payload" in each tree) is copied
to the GRUB source tree as ".config", then added to GRUB's
memdisk in the same way, as grub.cfg.

Several other design changes had to be made because of this:

* grub.cfg in memdisk no longer automatically jumps to one
  in CBFS, but now shows a menuentry for it if available

* Certain commands in script/trees are disabled for GRUB,
  such as *config make commands.

* gnulib is now defined in config/submodule/grub/, instead
  of config/git/grub - and this mitigates an existing bug
  where downloading gnulib first would make grub no longer
  possible to download in lbmk.

The coreboot option CONFIG_FINALIZE_USB_ROUTE_XHCI has been
re-enabled on: Dell OptiPlex 9020 MT, Dell OptiPlex 9020 SFF,
Lenovo ThinkPad T440p and Lenovo ThinkPad W541 - now USB should
work again in GRUB.

The GRUB payload has been re-enabled on HP EliteBook 820 G2.

This change will enable per-board GRUB optimisation in the
future. For example, we hardcode what partitions and LVMs
GRUB scans because * is slow on ICH7-based machines, due
to GRUB's design. On other machines, * is reasonably fast,
for automatically enumerating the list of devices for boot.

Use of * (and other wildcards) could enable our GRUB payload
to automatically boot more distros, with minimal fuss. This
can be done at a later date, in subsequent revisions.

Signed-off-by: Leah Rowe <leah@libreboot.org>

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