SPL is the name used for spl on i.MX6, so update the same on
BR2_TARGET_UBOOT_SPL_NAME help text.
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
[Thomas: rewrap Config.in help text.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>