efi_loader: Initial HII database protocols

This patch provides enough implementation of the following protocols to
run EDKII's Shell.efi and UEFI SCT:

  * EfiHiiDatabaseProtocol
  * EfiHiiStringProtocol

Not implemented are:
  * ExportPackageLists()
  * RegisterPackageNotify()/UnregisterPackageNotify()
  * SetKeyboardLayout() (i.e. *current* keyboard layout)

HII database protocol in this patch series can handle only:
  * GUID package
  * string package
  * keyboard layout package
  (The other packages, except Device path package, will be necessary
   for interactive and graphical UI.)

Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Alexander Graf <agraf@suse.de>