kconfig: move soc selection to ZEPHYR_BASE/soc/

Rather than do that for each architecture, source SoC Kconfigs where the
code is maintained, under ZEPHYR_BASE/soc.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

soc/Kconfig

@@ -0,0 +1,12 @@
+
+choice
+	prompt "SoC/CPU/Configuration Selection"
+
+source "soc/$ENV_VAR_ARCH/*/Kconfig.soc"
+
+endchoice
+
+menu "Hardware Configuration"
+osource "soc/$ENV_VAR_ARCH/*/Kconfig"
+
+endmenu