Most architectures uses /boot for there kernel image, so let this be reflected
by the kernel.
If INSTALL_PATH shell variable is set then this will have effect.
If INSTALL_PATH is set one the commanline to make like this:
make INSTALL_PATH=/nfs/boot install
then this will override both kbuild and shell variable.

If an arch prefer another default this must be set in the arch Makefile
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>