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/patches-5.10/100-bootargs.patch

@@ -0,0 +1,11 @@
+--- a/arch/arm/boot/dts/imx6dl-wandboard.dts
++++ b/arch/arm/boot/dts/imx6dl-wandboard.dts
+@@ -16,4 +16,8 @@
+ 		device_type = "memory";
+ 		reg = <0x10000000 0x40000000>;
+ 	};
++
++	chosen {
++		bootargs = "console=ttymxc0,115200";
++	};
+ };