MIPS: generic: Move Boston FIT image source to its own file

MIPS: generic: Move Boston FIT image source to its own file

Move the Boston board flattened image tree source into its own file
which is concatenated into the final image tree source used to build the
flattened image tree. Separating boards into different files will help
us to avoid conflicts as boards are added.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16939/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>