grub_git: remove armv7a from COMPATIBLE_HOST

grub_git: remove armv7a from COMPATIBLE_HOST

grub_git recipe supports arm architectures. However, when building against
armv7a arch, we would meet the following error.

| build-grub-module-verifier: error: unsupported relocation 0x2b.
| make[3]: *** [disk.mod] Error 1
| make[3]: *** Waiting for unfinished jobs....
| build-grub-module-verifier: error: unsupported relocation 0x2b.
| make[3]: *** [cat.mod] Error 1
| build-grub-module-verifier: error: unsupported relocation 0x2b.
| make[3]: *** [cmp.mod] Error 1

This address, along with 0x2c, is not supported yet, according to grub
upstream.

So remove armv7a from COMPATIBLE_HOST for now.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>