ARM: add 'uinstall' target for installing uboot kernels

We have 'install' and 'zinstall' for installing Image and zImage
kernels, so add 'uinstall' to complete the set.

This allows developers to have a ~/bin/installkernel script which (eg)
copies the kernel to the tftp server automatically once the kernel
has built, resulting in a better workflow.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>