efi: Move exit_boot_services into a function

efi: Move exit_boot_services into a function

At present this code is inline in the app and stub. But they do the same
thing. The difference is that the stub does it immediately and the app
doesn't want to do it until the end (when it boots a kernel) or not at
all, if returning to UEFI.

Move it into a function so it can be called as needed.

Add a comment showing how to store the memory map so that it can be
accessed within the app if needed, for debugging purposes only. The map
can change without notice.

Signed-off-by: Simon Glass <sjg@chromium.org>