cmd_pxe.c add any option for filesystem with sysboot uses generic load

Signed-off-by: Dennis Gilmore <dennis@ausil.us>
config: add config_distro_defaults.h

describe a set of default features that distros can rely on being available.
having this common definition means that distros can easily support systems
implementing them.

Signed-off-by: Dennis Gilmore <dennis@ausil.us>