u-boot.inc: allow variable make targets

u-boot.inc: allow variable make targets

* Later versions of u-boot sometimes have top level make targets
  such as u-boot.imx, u-boot.kwb, and for upcoming TI devices
  u-boot.ti.
* These targets define different calls to the mkimage tool to
  change the images created, such as changing the load address.
* This change allows recipes to change the default make target
  of "all" to something like u-boot.ti or u-boot.imx by setting
  the UBOOT_MAKE_TARGET variable.

Signed-off-by: Chase Maupin <Chase.Maupin@ti.com>
Signed-off-by: Koen Kooi <koen@openembedded.org>