boot/uboot: fix the help text of BR2_TARGET_UBOOT_ENVIMAGE_SOURCE

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>