ll_types.c: optimize link type table, make it conditional

ll_types.c: optimize link type table, make it conditional
            on FEATURE_IP_RARE_PROTOCOLS

function                                             old     new   delta
static.arphrd_name                                     -     373    +373
static.arphrd_type                                     -     124    +124
ll_type_n2a                                           58      78     +20
static.arphrd_names                                  496       -    -496
------------------------------------------------------------------------------
(add/remove: 2/1 grow/shrink: 1/0 up/down: 517/-496)           Total: 21 bytes
   text    data     bss     dec     hex filename
 776880     974    9420  787274   c034a busybox_old
 776594     974    9420  786988   c022c busybox_unstripped