backports: Makefile: introduce KROOT and fix INSTALL_MOD_PATH definition

INSTALL_MOD_PATH should point to the root directory of the target
rootfs, what is KROOT.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
backports: Makefile: introduce KVERSION and rework KLIB definition

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
backports: Makefile: export KROOT and KVERSION variables

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
backports: blacklist.sh: support install root not in /

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
backports: check_depmod.sh: support install root not in /

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
backports: Makefile.real: use depmod.sh instead of the depmod program

Call depmod.sh from the kernel source tree and set all parameter to
improve cross-compilation support.

This way, target root location, kernel version and symbol prefix are
also handled.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
backports: Makefile.real: disable update-initramfs.sh execution when cross-compiling

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
backports: Makefile.real: silent reboot message when cross-compiling

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
backports: compress_module.sh: gzip silently overrides existing compressed modules

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
backports: Makefile: update help text

Signed-off-by: Samuel Martin <s.martin49@gmail.com>