boot/uboot/Config.in: Fix BR2_TARGET_UBOOT_SPL

Building an SPL image depends on the board configuration. This option
does not enable the SPL build, but only copies the built SPL image to
the binary images folder. The current help text is misleading.

Signed-off-by: Jörg Krause <jkrause@posteo.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>