board: sifive: unmatched: set fdtfile with unquoted variable.

board: sifive: unmatched: set fdtfile with unquoted variable.

The fdtdfile variable contains quotes:

  printenv fdtfile
  fdtfile="sifive/hifive-unmatched-a00.dtb"

But this leads to issues which booting with an extlinux.conf format
file failing to find the .dtb file:

  Retrieving file: /usr/lib/linux-image-6.12.63+deb13-riscv64/"sifive/hifive-unmatched-a00.dtb"
  Skipping fdtdir /usr/lib/linux-image-6.12.63+deb13-riscv64/ for failure retrieving dts

Use the DEFAULT_FDT_FILE variable which has the quotes removed.

Signed-off-by: Vagrant Cascadian <vagrant@debian.org>