kernel-fitImage: Strip path component from dtb

kernel-fitImage: Strip path component from dtb

Machines that have added subdirectires to the KERNEL_DEVICETREE
recently, such as arm32 boards that were moved under subdirectories in
Linux 6.5, will have that subdirectory in the node name of the FIT. This
breaks existing systems that select a configuration in u-boot by it's
name.

Strip off the directory component from the device tree to preserve
compatibility.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Ninad Palsule <ninad@linux.ibm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>