tools: arm: imx: Implement BOOT_OFFSET command for imximage

Implement BOOT_OFFSET command for imximage. This command is parallel
to current BOOT_FROM command, but allows more flexibility in configuring
arbitrary image header offset. Also add an imximage.cfg with default
offset values into arm/arch/imx-common/ so the board-specific imximage.cfg
can include this file to avoid magic constants.

The syntax of BOOT_OFFSET command is "BOOT_OFFSET <u32 offset>".

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Cc: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tom Rini <trini@ti.com>
Acked-by: Stefano Babic <sbabic@denx.de>
Acked-by: Stefan Roese <sr@denx.de>
diff --git a/doc/README.imximage b/doc/README.imximage
index 073e3fc..802eb90 100644
--- a/doc/README.imximage
+++ b/doc/README.imximage
@@ -65,9 +65,27 @@
 				This command need appear the fist before
 				other valid commands in configuration file.
 
+	BOOT_OFFSET		value
+
+				This command is parallel to BOOT_FROM and
+				is preferred over BOOT_FROM.
+
+				value:  Offset of the image header, this
+					value shall be set to one of the
+					values found in the file:
+						arch/arm/include/asm/\
+						imx-common/imximage.cfg
+				Example:
+				BOOT_OFFSET FLASH_OFFSET_STANDARD
+
 	BOOT_FROM		nand/spi/sd/onenand/nor/sata
+
+				This command is parallel to BOOT_OFFSET and
+				is to be deprecated in favor of BOOT_OFFSET.
+
 				Example:
 				BOOT_FROM spi
+
 	DATA			type address value
 
 				type: word=4, halfword=2, byte=1