cmd: bootefi: move library interfaces under lib/efi_loader

In the prior commits, interfaces for executing EFI binary and boot manager
were carved out. Move them under efi_loader directory so that they can
be called from other places without depending on bootefi command.

Only efi_selftest-related code will be left in bootefi.c.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>