ldlinux: Sprinkle const type qualifers

ldlinux: Sprinkle const type qualifers

The following warnings are all caused because we're assigning from
const char * to char *. As the strings are never modified, it makes
sense to add the const type qualifer instead of casting away const.

readconfig.c: In function ‘parse_config_file’:
readconfig.c:708:11: warning: assignment discards qualifiers from pointer target type
readconfig.c:937:10: warning: assignment discards qualifiers from pointer target type
readconfig.c:987:20: warning: assignment discards qualifiers from pointer target type
readconfig.c:998:15: warning: assignment discards qualifiers from pointer target type
readconfig.c:1004:18: warning: assignment discards qualifiers from pointer target type
readconfig.c:1041:11: warning: assignment discards qualifiers from pointer target type

Signed-off-by: Matt Fleming <matt.fleming@linux.intel.com>