kbuild: default value for INSTALL_PATH set to /boot

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>

Makefile

@@ -545,10 +545,8 @@ export KBUILD_IMAGE ?= vmlinux
 
 #
 # INSTALL_PATH specifies where to place the updated kernel and system map
-# images.  Uncomment if you want to place them anywhere other than root.
-#
-
-#export	INSTALL_PATH=/boot
+# images. Default is /boot, but you can set it to other values
+export	INSTALL_PATH ?= /boot
 
 #
 # INSTALL_MOD_PATH specifies a prefix to MODLIB for module directory