dts: re-write dts/Makefile more simply with Kbuild

Useful rules in scripts/Makefile.lib allows us to easily
generate a device tree blob and wrap it in assembly code.

We do not need to parse a linker script to get output format and arch.

This commit deletes ./u-boot.dtb since it is a copy of dts/dt.dtb.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

.gitignore

@@ -48,7 +48,6 @@
 /u-boot.lds
 /u-boot.ubl
 /u-boot.ais
-/u-boot.dtb
 /u-boot.sb
 
 #