use auto_string() where appropriate to kill a few statics

use auto_string() where appropriate to kill a few statics

Custom linker script 'busybox_ldscript' found, using it
function                                             old     new   delta
static.str                                             4       -      -4
static.passwd                                          4       0      -4
bb_ask                                               322     311     -11
ether_print                                           63      47     -16
UNSPEC_print                                          82      66     -16
INET_sprint                                           59      38     -21
INET6_sprint                                          54      30     -24
make_human_readable_str                              292     235     -57
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 0/7 up/down: 0/-153)           Total: -153 bytes
   text	   data	    bss	    dec	    hex	filename
 939880	    992	  17480	 958352	  e9f90	busybox_old
 939736	    992	  17456	 958184	  e9ee8	busybox_unstripped

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