Add the required typedefs etc for using con_in's simple text input
protocol, and for using the boottime event services.

Also add the prototype for the "stall" boot service.
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200518190716.751506-19-nivedita@alum.mit.eduSigned-off-by: Ard Biesheuvel <ardb@kernel.org>