efi_loader: rename BOOTEFI_BOOTMGR to EFI_BOOTMGR

At this point, EFI boot manager interfaces is fully independent from
bootefi command. So just rename the configuration parameter.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>