imx8mm-evk: Generate a single bootable flash.bin again

After the conversion to binman in commit 8996e6b7c6 ("imx8mm_evk: switch
to use binman to pack images"), it is necessary to flash both flash.bin and
u-boot.itb to get a bootable system. Prior to this commit, only flash.bin
was needed.

Such new requirement breaks existing distro mechanisms to generate the
final binary because the extra u-boot.itb is now required.

Generate a final flash.bin that can be used again as a single
bootable binary to keep the original behavior.

After this change the SPL binary is called spl.bin, which is a more
descriptive name for its purpose, and can still be used standalone
(for example, for secure boot purposes).

Also update imx8mm_evk.rst to remove the u-boot.itb copy step.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Heiko Thiery <heiko.thiery@gmail.com>

arch/arm/dts/imx8mm-evk-u-boot.dtsi

@@ -150,7 +150,9 @@
 	};
 
 
-       flash {
+	spl {
+		filename = "spl.bin";
+
 		mkimage {
 			args = "-n spl/u-boot-spl.cfgout -T imx8mimage -e 0x7e1000";
 
@@ -217,4 +219,19 @@
 			};
 		};
 	};
+
+	imx-boot {
+		filename = "flash.bin";
+		pad-byte = <0x00>;
+
+		spl: blob-ext@1 {
+			offset = <0x0>;
+			filename = "spl.bin";
+		};
+
+		uboot: blob-ext@2 {
+			offset = <0x57c00>;
+			filename = "u-boot.itb";
+		};
+	};
 };

board/freescale/imx8mm_evk/imximage-8mm-lpddr4.cfg

@@ -6,4 +6,4 @@
 #define __ASSEMBLY__
 
 BOOT_FROM	sd
-LOADER		mkimage.flash.mkimage	0x7E1000
+LOADER		u-boot-spl-ddr.bin	0x7E1000

doc/board/nxp/imx8mm_evk.rst

@@ -50,7 +50,6 @@ Burn the flash.bin to MicroSD card offset 33KB:
 .. code-block:: bash
 
    $sudo dd if=flash.bin of=/dev/sd[x] bs=1024 seek=33 conv=notrunc
-   $sudo dd if=u-boot.itb of=/dev/sdc bs=1024 seek=384 conv=sync
 
 Boot
 ----