uboot: install multiple spl images

uboot: install multiple spl images

For some platforms, there are multiple generated spl images.  Extend
BR2_TARGET_UBOOT_SPL_NAME to allow these multiple images to be installed
after uboot build completes.

For example, the NextThingCo C.H.I.P. uses two binaries from uboot,
spl/sunxi-spl.bin and spl/sunxi-spl-with-ecc.bin.

Signed-off-by: Jason Abele <jason@nextthing.co>
[Maxime:
 - Add foreach loop for general case and mkpimage
 - Use firstword for zynq case]
Signe-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>