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/common/board_f.c b/common/board_f.c
index aa70c3e..d0ee6f7 100644
--- a/common/board_f.c
+++ b/common/board_f.c
@@ -360,7 +360,7 @@
 {
 #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 */
 # ifdef CONFIG_SYS_SYM_OFFSETS