MIPS: DTS: Fix build errors used with various configs

If CONFIG_MIPS_MALTA is not set but CONFIG_LEGACY_BOARD_SEAD3 is set,
the subdir arch/mips/boot/dts/mti will not be built, so the sead3.dts
which depends on CONFIG_LEGACY_BOARD_SEAD3 in this subdir is also not
built, and then there exists the following build error, fix it.

  LD      .tmp_vmlinux.kallsyms1
arch/mips/generic/board-sead3.o:(.mips.machines.init+0x4): undefined reference to `__dtb_sead3_begin'
Makefile:1106: recipe for target 'vmlinux' failed
make: *** [vmlinux] Error 1

Additionally, add CONFIG_FIT_IMAGE_FDT_BOSTON check for subdir img to
fix the following build error when CONFIG_MACH_PISTACHIO is not set but
CONFIG_FIT_IMAGE_FDT_BOSTON is set.

FATAL ERROR: Couldn't open "boot/dts/img/boston.dtb": No such file or directory

Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Fixes: 41528ba6 ("MIPS: DTS: Only build subdir of current platform")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>