kbuild: Create INSTALL_PATH directory if it does not exist

If INSTALL_PATH is not a valid directory, create it, like what
modules_install and dtbs_install will do in the same situation.

Signed-off-by: Zhang Bingwu <[e-mail]>
Reviewed-by: Nicolas Schier <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>

scripts/install.sh

···
 	fi
 done
 
+if [ -n "${INSTALL_PATH}" ] && ! [ -e "${INSTALL_PATH}" ]; then
+	mkdir -p "${INSTALL_PATH}"
+fi
+
 # User/arch may have a custom install script
 for file in "${HOME}/bin/${INSTALLKERNEL}"		\
 	    "/sbin/${INSTALLKERNEL}"			\