soc-family.inc: Add a default SOC_FAMILY value

Otherwise, if MACHINEOVERRIDES is expanded before SOC_FAMILY is set (which may happen as MACHINEOVERRIDES is included in OVERRIDES) we can see:

ExpansionError: Failure expanding variable MACHINEOVERRIDES, expression was
${@['', '${SOC_FAMILY}:']['${SOC_FAMILY}' != '']}p1022ds
which triggered exception SyntaxError: EOL while scanning string literal (MACHINEOVERRIDES, line 1)

To avoid this, give SOC_FAMILY a default empty value so it doesn't get read as None.

index 0251da0..1ff0899 100644
@@ -1,2 +1,3 @@
 # Add SOC_FAMILY to machine overrides so we get access to e.g. 'omap3' and 'ti335x'
+SOC_FAMILY ??= ""
 MACHINEOVERRIDES =. "${@['', '${SOC_FAMILY}:']['${SOC_FAMILY}' != '']}"