uboot: qstrip BR2_TARGET_UBOOT_CUSTOM_DTS_PATH

We need to avoid failing of copy of custom dts using cp -f <>
command. So, just qstrip the variable as is done for other
configuration options.

Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Tested-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>