Clean up qemuarm-efi kernel config

Current qemuarm-efi defconfig contains many useless features,
remove it and use the qemuarm base config with additional EFI features.

For linux-yocto compatibility, set qemuarm-efi to fetch from the same
machine specific branches as qemuarm.

Signed-off-by: Luca Seritan <luca.seritan@huawei.com>