vsprintf.c: add GUID printing

This works (roughly) the same way as linux's, but we currently always
print lower-case (ie. we just keep %pUB and %pUL for compat with linux),
mostly just because that is what uuid_bin_to_str() supports.

  %pUb:   01020304-0506-0708-090a-0b0c0d0e0f10
  %pUl:   04030201-0605-0807-090a-0b0c0d0e0f10

It will be used by a later efi_loader paths for efi variables and for
device-path-to-text protocol, and also quite useful for debug prints
of protocol GUIDs.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>