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 <[e-mail]>
Reported-by: Guenter Roeck <[e-mail]>
Fixes: 41528ba6afe6 ("MIPS: DTS: Only build subdir of current platform")
Signed-off-by: Tiezhu Yang <[e-mail]>
Signed-off-by: Thomas Bogendoerfer <[e-mail]>

arch/mips/boot/dts/Makefile

···
 subdir-$(CONFIG_BMIPS_GENERIC)		+= brcm
 subdir-$(CONFIG_CAVIUM_OCTEON_SOC)	+= cavium-octeon
 subdir-$(CONFIG_MACH_PISTACHIO)		+= img
+subdir-$(CONFIG_FIT_IMAGE_FDT_BOSTON)	+= img
 subdir-$(CONFIG_MACH_INGENIC)		+= ingenic
 subdir-$(CONFIG_LANTIQ)			+= lantiq
 subdir-$(CONFIG_MACH_LOONGSON64)	+= loongson
 subdir-$(CONFIG_MSCC_OCELOT)		+= mscc
 subdir-$(CONFIG_MIPS_MALTA)		+= mti
+subdir-$(CONFIG_LEGACY_BOARD_SEAD3)	+= mti
 subdir-$(CONFIG_NLM_XLP_BOARD)		+= netlogic
 subdir-$(CONFIG_FIT_IMAGE_FDT_NI169445)	+= ni
 subdir-$(CONFIG_MACH_PIC32)		+= pic32