MAKEALL: update to work with new boards.cfg syntax

Update MAKEALL to handle the optional SPL CPU field that was added to
boards.cfg.  This impacts the cases in MAKEALL that have to match
against CPU type (field 3).  In these cases use ':' as a field
separator to split the u-boot CPU from the SPL CPU.

Signed-off-by: Allen Martin <amartin@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>