discover/grub2: Fixes for bison 3.x

YYLEX_PARAM is removed in bison 3.0, so we need to pass the scanner
param directly through yyparse (rather than referencing
parser->scanner). Unfortunately, we don't have the lexer header
available at the time we declare yyparse, so we need to pass a void *
here.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>