Grub2 efi files

[Johannes Kliemann]

Hi Alex,

how have you built the efi files for grub2? I tried to build them from
master and from the commit you used in Genode [1].
If I run configure with --with-platform=efi some headers are missing for
the build and if I configure without it grub-mkimage fails with error:
no symbol table.

Regards,

JK

[1]:
https://github.com/alex-ab/genode/commit/de98d57a85c3b1f5fd632f1dd14f2984867fd852