grub2: install in $(HOST_DIR) instead of $(HOST_DIR)/usr

grub2 builds for the target but installs with DESTDIR=$(HOST_DIR). Since
we set prefix to /usr in TARGET_CONF_OPTS, this results in installing
things in $(HOST_DIR)/usr.

To make sure we don't install in $(HOST_DIR)/usr, override --prefix and
--exec-prefix.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

boot/grub2/grub2.mk

@@ -63,6 +63,8 @@ GRUB2_CONF_ENV = \
 GRUB2_CONF_OPTS = \
 	--target=$(GRUB2_TARGET) \
 	--with-platform=$(GRUB2_PLATFORM) \
+	--prefix=/ \
+	--exec-prefix=/ \
 	--disable-grub-mkfont \
 	--enable-efiemu=no \
 	ac_cv_lib_lzma_lzma_code=no \