lib: Define autoboot_options, device_type helpers

Add the new autoboot_option struct, and helper functions for working
with device_type enums. device_type_name() returns exact strings as used
by platform code to read/write nvram params, so
device_type_display_name() is added for use in user-visible strings.

Signed-off-by: Samuel Mendoza-Jonas <sam.mj@au1.ibm.com>