Change the fdt_static_dtb.S dependency

fdt_static_dtb.S dependency in sys/conf/files is currently set as:
$S/boot/fdt/dts/${MACHINE}/${FDT_DTS_FILE}

This is wrong, as what fdt_static_dtb.S actually uses is the DTB file
produced from the FDT_DTS_FILE.
In addition it also makes using DTS files stored in $S/gnu/dts/${MACHINE}/
impossible.

So, change the dependency to "fdt_dtb_file", which seems to be the right
option here anyway.

Approved by: adrian (mentor)
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D5963