grub_git: set COMPATIBLE_HOST_armv7a to null

grub_git: set COMPATIBLE_HOST_armv7a to null

It doesn't work with armv7a:
| build-grub-module-verifier: error: unsupported relocation 0x2b.
| make[3]: *** [reboot.mod] Error 1
| make[3]: *** Waiting for unfinished jobs....
| build-grub-module-verifier: error: unsupported relocation 0x2b.
| build-grub-module-verifier: error: unsupported relocation 0x2b.
| make[3]: *** [halt.mod] Error 1
| make[3]: *** [cat.mod] Error 1
| build-grub-module-verifier: error: unsupported relocation 0x2b.
| build-grub-module-verifier: error: unsupported relocation 0x2b.
| build-grub-module-verifier: error: unsupported relocation 0x2b.
| make[3]: *** [disk.mod] Error 1
| make[3]: *** [gptsync.mod] Error 1
| make[3]: *** [eval.mod] Error 1
| build-grub-module-verifier: error:build-grub-module-verifier: error:  unsupported relocation 0x2bunsupported relocation 0x2b.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>