Cleanup of UEFI code

mk/aarch64-unknown-uefi.mk

src/os/uefi/arch/aarch64/mod.rs

src/os/uefi/arch/aarch64/partitions.rs

src/os/uefi/arch/x86_64/mod.rs

src/os/uefi/arch/x86_64/partitions.rs

src/os/uefi/mod.rs

src/os/uefi/null.rs