boot/uboot: fix the help text of BR2_TARGET_UBOOT_ENVIMAGE_SOURCE

In commit 2c8ff251cb16d38fb417a488ee0367cd03eebda2 ("boot/uboot: add
option to generate env image from default env"), the possibility of
generating an environment image using the built-in U-Boot environment
as a source was added.

This is meant to happen when the string option
BR2_TARGET_UBOOT_ENVIMAGE_SOURCE is empty, but the original commit
added the Config.in help text to BR2_TARGET_UBOOT_ENVIMAGE, which is a
boolean option, and therefore cannot be empty.

Move the help text to BR2_TARGET_UBOOT_ENVIMAGE_SOURCE, where it
belongs.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit f9ab00ba4606b46ac6114a634dc93cf3ee76a108)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/uboot/Config.in

@@ -487,8 +487,6 @@ menuconfig BR2_TARGET_UBOOT_ENVIMAGE
 
 	  The environment image will be called uboot-env.bin.
 
-	  Leave empty to generate image from compiled-in env.
-
 if BR2_TARGET_UBOOT_ENVIMAGE
 
 config BR2_TARGET_UBOOT_ENVIMAGE_SOURCE
@@ -500,6 +498,8 @@ config BR2_TARGET_UBOOT_ENVIMAGE_SOURCE
 
 	  Multiple source files are concatenated in the order listed.
 
+	  Leave empty to generate image from compiled-in env.
+
 config BR2_TARGET_UBOOT_ENVIMAGE_SIZE
 	string "Size of environment"
 	help