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/include/common.h b/include/common.h
index 221b776..672c0b5 100644
--- a/include/common.h
+++ b/include/common.h
@@ -302,7 +302,7 @@
 int	last_stage_init(void);
 extern ulong monitor_flash_len;
 int mac_read_from_eeprom(void);
-extern u8 _binary_dt_dtb_start[];	/* embedded device tree blob */
+extern u8 __dtb_dt_begin[];	/* embedded device tree blob */
 int set_cpu_clk_info(void);
 int print_cpuinfo(void);
 int update_flash_size(int flash_size);