grub2: really disable liblzma

There is no option --enable-liblzma=no in grub2's configure script, so
the only way to disable liblzma support is to pass
ac_cv_lib_lzma_lzma_code=no.

Signed-off-by: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Thomas: expand commit log, as suggested by Yann E. Morin.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

boot/grub2/grub2.mk

@@ -62,7 +62,7 @@ GRUB2_CONF_OPTS = \
 	--with-platform=$(GRUB2_PLATFORM) \
 	--disable-grub-mkfont \
 	--enable-efiemu=no \
-	--enable-liblzma=no \
+	ac_cv_lib_lzma_lzma_code=no \
 	--enable-device-mapper=no \
 	--enable-libzfs=no \
 	--disable-werror