efi_loader: sanity checks when freeing memory

Use a checksum to validate that efi_free_pool() is only called for memory
allocated by efi_allocated_pool().

Add a plausibility check to efi_free_pages() checking that the address
passed is page aligned.

Update related function comments to match Sphinx style.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>