efi_loader: split unrelated code from efi_bootmgr.c

Some code moved from cmd/bootefi.c is actually necessary only for "bootefi
<addr>" command (starting an image manually loaded by a user using U-Boot
load commands or other methods (like JTAG debugger).

The code will never been opted out as unused code by a compiler which
doesn't know how EFI boot manager is implemented. So introduce a new
configuration, CONFIG_EFI_BINARY_EXEC, to enforce them opted out
explicitly.

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