Fix use of SOC_FAMILY as the most right value is preferred

The SOC_FAMILY uses the MACHINEOVERRIDES and it evaluates from left to
right. In this case, the most SoC specific value needs to be kept in
the right so it take precedence.

This has been catch when finding out why i.MX6SL EVK where using the
wrong value as entry point, and this was the root cause.

Change-Id: I34ae0076690716ba71fd80311a1e427821e4e762
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>