Currently, the (z/u)install targets in arch/arm/Makefile descend into
arch/arm/boot/Makefile to invoke the shell script, but there is no
good reason to do so.

arch/arm/Makefile can run the shell script directly.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>