hex: Replace rf_ prefix with hex_

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>
hex: Add hex_get_byte() function.

hex_get_byte() is a helper function to help decode hexdumps. It uses
a context pointer to allow it to work a byte at a time.

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>