ipq40xx: arm: compressed: add appended DTB section

This adds a appended_dtb section to the ARM decompressor
linker script.

This allows using the existing ARM zImage appended DTB support for
appending a DTB to the raw ELF kernel.

Its size is set to 1MB max to match the zImage appended DTB size limit.

To use it to pass the DTB to the kernel, objcopy is used:

objcopy --set-section-flags=.appended_dtb=alloc,contents \
	--update-section=.appended_dtb=<target>.dtb vmlinux

This is based off the following patch:
c063e27e02

Signed-off-by: Robert Marko <robimarko@gmail.com>

target/linux/ipq40xx/patches-5.4/301-arm-compressed-add-appended-DTB-section.patch

@@ -0,0 +1,48 @@
+From 0843a61d6913bdac8889eb048ed89f7903059787 Mon Sep 17 00:00:00 2001
+From: Robert Marko <robimarko@gmail.com>
+Date: Fri, 30 Oct 2020 13:36:31 +0100
+Subject: [PATCH] arm: compressed: add appended DTB section
+
+This adds a appended_dtb section to the ARM decompressor
+linker script.
+
+This allows using the existing ARM zImage appended DTB support for
+appending a DTB to the raw ELF kernel.
+
+Its size is set to 1MB max to match the zImage appended DTB size limit.
+
+To use it to pass the DTB to the kernel, objcopy is used:
+
+objcopy --set-section-flags=.appended_dtb=alloc,contents \
+	--update-section=.appended_dtb=<target>.dtb vmlinux
+
+This is based off the following patch:
+https://github.com/openwrt/openwrt/commit/c063e27e02a9dcac0e7f5877fb154e58fa3e1a69
+
+Signed-off-by: Robert Marko <robimarko@gmail.com>
+---
+ arch/arm/boot/compressed/vmlinux.lds.S | 9 ++++++++-
+ 1 file changed, 8 insertions(+), 1 deletion(-)
+
+--- a/arch/arm/boot/compressed/vmlinux.lds.S
++++ b/arch/arm/boot/compressed/vmlinux.lds.S
+@@ -93,6 +93,13 @@ SECTIONS
+ 
+   _edata = .;
+ 
++  .appended_dtb : {
++    /* leave space for appended DTB */
++    . += 0x100000;
++  }
++
++  _edata_dtb = .;
++
+   /*
+    * The image_end section appears after any additional loadable sections
+    * that the linker may decide to insert in the binary image.  Having
+@@ -132,4 +139,4 @@ SECTIONS
+   .stab.indexstr 0	: { *(.stab.indexstr) }
+   .comment 0		: { *(.comment) }
+ }
+-ASSERT(_edata_real == _edata, "error: zImage file size is incorrect");
++ASSERT(_edata_real == _edata_dtb, "error: zImage file size is incorrect");