of/fdt: consolidate built-in dtb section variables

Unify the various architectures __dtb_start and __dtb_end definitions
moving them into of_fdt.h.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Vineet Gupta <vgupta@synopsys.com>
Acked-by: James Hogan <james.hogan@imgtec.com>
Tested-by: Michal Simek <michal.simek@xilinx.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Jonas Bonn <jonas@southpole.se>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: linux-metag@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: linux@lists.openrisc.net
Cc: linux-xtensa@linux-xtensa.org
Tested-by: Grant Likely <grant.likely@linaro.org>
Tested-by: Stephen Chivers <schivers@csc.com>

include/linux/of_fdt.h

@@ -80,6 +80,9 @@ extern int __initdata dt_root_addr_cells;
 extern int __initdata dt_root_size_cells;
 extern struct boot_param_header *initial_boot_params;
 
+extern char __dtb_start[];
+extern char __dtb_end[];
+
 /* For scanning the flat device-tree at boot time */
 extern char *find_flat_dt_string(u32 offset);
 extern int of_scan_flat_dt(int (*it)(unsigned long node, const char *uname,