spl: allow boot from first bootable partition

This was implemented in order to get dual-slot bootloader
partitions on the BeagleBone Black, whose MLO boots from
the first bootable partition: MLO chainloads u-boot in the
same way.

Signed-off-by: Jérôme Carretero <cJ-uboot@zougloub.eu>
Reviewed-by: Tom Rini <trini@konsulko.com>