image_types_fsl: fix for dtb filenames containing multiple dots
In _generate_boot_image(), if a dtb file has more than one dot the
current logic fails. e.g. 'device.tree.dtb' will be truncated at the
first dot to 'device' while it should be 'device.tree'.
Fix by removing awk in favour of letting basename also remove
the .dtb extension.
Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com> Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>