boot/uboot/uboot.mk: Strip quotes from BR2_TARGET_UBOOT_SPL_NAME

Remove the quotes from the BR2_TARGET_UBOOT_SPL_NAME variable.

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