common: grub build scripts for 32bit arm

Adds a script for building Grub package for a 32bit Arm target.
Adds a specific Grub configuration file.
Grub also needs a dedicated patch unless what the EFI memory management
layer confuses reserved internal RAM and system main memory ending
i a bad computation of the main memory base address.

Signed-off-by: Christophe Priouzeau <christophe.priouzeau@foss.st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>