imx6: rename target to 'imx'

This is first step in migrating to a generic i.MX target which in the
next steps will also get divided into arch-specific subtargets.

In the result, this will make it possible to support, within a single
target, also other modern NXP i.MX families, like the i.MX 7, i.MX 8
or recently introduced i.MX 9.

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>

target/linux/imx/profiles/100-default.mk

@@ -0,0 +1,14 @@
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# Copyright (C) 2013 OpenWrt.org
+
+define Profile/Default
+  PRIORITY:=1
+  NAME:=Default Profile
+endef
+
+define Profile/Default/Description
+ Package set compatible with most NXP i.MX based boards.
+endef
+
+$(eval $(call Profile,Default))