[bzimage] Fix parsing of "vga=..." when not at end of command line

bzimage_parse_cmdline() uses strcmp() to identify the named "vga=..."
kernel command line option values, which will give a false negative if
the option is not last on the command line.

Fix by temporarily changing the relevant command line separator (if
any) to a NUL terminator.

Debugged-by: Simon Rettberg <simon.rettberg@rz.uni-freiburg.de>
Signed-off-by: Michael Brown <mcb30@ipxe.org>