efi/libstub: Fix command line fallback handling when loading files

efi/libstub: Fix command line fallback handling when loading files

CONFIG_CMDLINE, when set, is supposed to serve either as a fallback when
no command line is provided by the bootloader, or to be taken into account
unconditionally, depending on the configured options.

The initrd and dtb loader ignores CONFIG_CMDLINE in either case, and
only takes the EFI firmware provided load options into account. This
means that configuring the kernel with initrd= or dtb= on the built-in
command line does not produce the expected result.

Fix this by doing a separate pass over the built-in command line when
dealing with initrd= or dtb= options.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>