Split common routines from kboot parser

Pull out the common .conf file parsing logic from kboot-parser.c and
into two new files parser-conf.h and parser-conf.c, and rework
the kboot parser to use those common routines.

The new common routines are based on a .conf file parser context
struct conf_context.  The specific parsers setup the context then
call the main parsing entry routine conf_parse().  conf_parse()
uses the context info to open and read .conf files and call
parser specific callbacks to process name:value pairs and to
add boot_option instances to the discover server.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>