efi/libstub: Parse builtin command line after bootloader provided one

efi/libstub: Parse builtin command line after bootloader provided one

When CONFIG_CMDLINE_EXTEND is set, the core kernel command line handling
logic appends CONFIG_CMDLINE to the bootloader provided command line.
The EFI stub does the opposite, and parses the builtin one first.

The usual behavior of command line options is that the last one takes
precedence if it appears multiple times, unless there is a meaningful
way to combine them. In either case, parsing the builtin command line
first while the core kernel does it in the opposite order is likely to
produce inconsistent results in such cases.

Therefore, switch the order in the stub to match the core kernel.

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