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>
diff --git a/arch/microblaze/lib/board.c b/arch/microblaze/lib/board.c
index 59956a8..fafeeae 100644
--- a/arch/microblaze/lib/board.c
+++ b/arch/microblaze/lib/board.c
@@ -87,7 +87,7 @@
 
 #ifdef CONFIG_OF_EMBED
 	/* Get a pointer to the FDT */
-	gd->fdt_blob = _binary_dt_dtb_start;
+	gd->fdt_blob = __dtb_dt_begin;
 #elif defined CONFIG_OF_SEPARATE
 	/* FDT is at end of image */
 	gd->fdt_blob = (void *)__end;