Convert SOC_FAMILY to MACHINEOVERRIDES

Convert SOC_FAMILY to MACHINEOVERRIDES

The SOC_FAMILY is an indirection which makes the understanding, from
OpenEmbedded versed developers and users, more difficult. Instead of
using the SOC_FAMILY indirection which is rarely used, we now move to
the MACHINEOVERRIDES mechanism as this is in fact what is in use.

Change-Id: Ic6e58b79bdc990ac853c87761c3fff635786e4e5
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>