LinuxKPI: add eth_random_addr() and device_get_mac_address()

Add eth_random_addr() and a dummy of device_get_mac_address()
pending OF (FDT) support needed by drivers.

While here remove a white space in random_ether_addr().

MFC after: 3 days
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D34228