ldlinux: Pass config filename as argv[1] to ldlinux.c32

ldlinux: Pass config filename as argv[1] to ldlinux.c32

Instead of hijacking ConfigName use a more standard method of passing
a config name to ldlinux.c32's main() function, via argc and argv.

This allows us to actually call open_config() the first time
ldlinux.c32 is executed even if the file system has already modified
ConfigName. For example, pxelinux_configfile() parses the DHCP 209
option and fills out ConfigName before ldlinux.c32 is launched, but
because the PXE code needs to do things with the path to the config
file (such as parsing the DHCP 210 option), we need to leave the
config mangling to open_config() and not try and lookup ConfigName
from ldlinux.

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