README: Drop information about commands

Most of this is duplicated in Kconfig help. Add some of that which is not,
and remove the help from the README.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
diff --git a/cmd/fastboot/Kconfig b/cmd/fastboot/Kconfig
index 89b9e73..b84529a 100644
--- a/cmd/fastboot/Kconfig
+++ b/cmd/fastboot/Kconfig
@@ -18,6 +18,8 @@
 	  protocol for downloading images, flashing and device control
 	  used on Android devices.
 
+	  See doc/README.android-fastboot for more information.
+
 config ANDROID_BOOT_IMAGE
 	bool "Enable support for Android Boot Images"
 	help