efi: Update README.efi to clarify build and test instructions

The doc has a misleading 'make menuconfig' when building the EFI
application and payload. Clarify this and also update information
on test with QEMU.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
1 file changed