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>