doc: android: Add documentation for Android Boot Image

Describe Android Boot Image format, how its support is implemented in
U-Boot and associated commands usage.

Signed-off-by: Sam Protsenko <joe.skb7@gmail.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 4734da1..6a2f6fb 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -394,6 +394,8 @@
 	  images contained in boot.img, like kernel, ramdisk, dtb, etc, and
 	  obtain corresponding meta-information from boot.img.
 
+	  See doc/android/boot-image.rst for details.
+
 config CMD_ELF
 	bool "bootelf, bootvx"
 	default y