tools/quilt: fix build error during kernel_menuconfig on a cleaned env

When running "make kernel_menuconfig" right after "make clean", we error
out with the following message:

Makefile:22: recipe for target '.../build_dir/target-arm_cortex-a7_musl-1.1.10_eabi/linux-.../linux-3.18.18/.quilt_checked' failed
make[2]: [.../build_dir/target-arm_cortex-a7_musl-1.1.10_eabi/linux-.../linux-3.18.18/.quilt_checked]
Error 127 (ignored)

The root cause of the problem is located in the OpenWrt specific quilt
patch, which sets the quilt install directory to $STAGING_DIR/../host.
However, on a clean environment (right after running "make clean"),
$STAGING_DIR doesn't exist. The "quilt" executable doesn't find its
different commands, errors out, and creates this message.

We're fixing it by using $STAGING_DIR_HOST rather than $STAGING_DIR,
which is absolute. It will work even if there is no target directory
yet.

Signed-off-by: Mathieu Olivari <mathieu@codeaurora.org>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 47410