grub2: only available on x86 and PowerPC and mark as BROKEN

The build process of grub2 breaks the compilation. It breaks with:

./configure: line 4766: syntax error near unexpected token `external'
./configure: line 4766: `AM_GNU_GETTEXT(external)'

In addition to this, it later requires Ruby. Do we really want to make
Buildroot depend on Ruby being installed on the host ? Do we really
want to build our own Ruby ? Do we even care about Grub2 ?

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>