u-boot.inc: Add UBOOT_BINARY sym links for UBOOT_CONFIG types

u-boot.inc: Add UBOOT_BINARY sym links for UBOOT_CONFIG types

An additional use case of UBOOT_CONFIG is when a machine has applicability
to boards of the same architecture but different in other ways
to require a different UBOOT_BINARY build.

The UBOOT_CONFIG default value can be a list of these board types.

For example:
    UBOOT_CONFIG ??= "boardA boardB"
    UBOOT_CONFIG[boardA] = "boardA_defconfig"
    UBOOT_CONFIG[boardB] = "boardB_defconfig"

Change do_install and do_deploy sections which process a UBOOT_CONFIG list
to create short symbolic links to each of the config types for UBOOT_BINARY.
This is similar to the links currently being created for
SPL_BINARY when it is defined with a UBOOT_CONFIG list.

For the above example, and UBOOT_BINARY as u-boot.bin,
the additional symbolic links created in the DEPLOYDIR would be

    u-boot.bin-boardA
    u-boot.bin-boardB

Signed-off-by: Lee Nipper <lee.nipper@linux.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>