mdev: add support for $DEVNAME and /dev/mdev.log debug aid

mdev: add support for $DEVNAME and /dev/mdev.log debug aid

function                                             old     new   delta
make_device                                         1843    2083    +240
mdev_main                                            712     804     +92
packed_usage                                       29236   29251     +15
keywords                                             288      19    -269
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/1 up/down: 347/-269)           Total: 78 bytes
   text    data     bss     dec     hex filename
 887506     497    7584  895587   daa63 busybox_old
 887982     497    7584  896063   dac3f busybox_unstripped

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>