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>

boot/grub2/Config.in

@@ -1,4 +1,9 @@
 config BR2_TARGET_GRUB2
 	bool "grub2"
+	# Only supports PC, Mac (Power, x86 and x86-64), Pegasos
+	depends on BR2_i386 || BR2_powerpc
+	# Build process is broken (./configure doesn't work, Ruby
+	# required, etc.)
+	depends on BROKEN
 	help
 	  The GRand Unified Bootloader for x86 systems, take #2.