net: Fix build regression in cmd_pxe.c

Not all boards define an SOC.  As a result, we can't depend on that.

This was introduced in 39f985536d3f0df5dba32c15b64ba2b5d32dd296

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>

common/cmd_pxe.c

@@ -27,7 +27,9 @@
 #define MAX_TFTP_PATH_LEN 127
 
 const char *pxe_default_paths[] = {
+#ifdef CONFIG_SYS_SOC
 	"default-" CONFIG_SYS_ARCH "-" CONFIG_SYS_SOC,
+#endif
 	"default-" CONFIG_SYS_ARCH,
 	"default",
 	NULL