ldlinux: Fixup lwip merge botch

ldlinux: Fixup lwip merge botch

The lwip merge changed the code for copying cmdline arguments from,

    strcpy(q, p);

to this,

    do {
	*q++ = ch = *p++;
    } while (ch);

which means 'p' no longer points at 'args'. Use 'args' explicitly for
the case where we need to apply a filename extension, otherwise users
end up seeing errors like,

    Failed to load COM32 file .c32

Reported-by: Mattias Schlenker <ms@mattiasschlenker.de>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>