rp2040: patch elf to checksum 2nd stage boot

targets/rp2040.ld

@@ -1,9 +1,29 @@
 
-MEMORY
-{
-    RAM (rwx)       : ORIGIN = 0x20000000, LENGTH = 256k
-}
-
 _stack_size = 2K;
 
+SECTIONS
+{
+    /* Second stage bootloader is prepended to the image. It must be 256 bytes
+       and checksummed.  The gap to the checksum is zero-padded.
+    */
+    .boot2 : {
+        __boot2_start__ = .;
+        KEEP (*(.boot2));
+
+        /* Explicitly allocate space for CRC32 checksum at end of second stage
+           bootloader
+        */
+        . = __boot2_start__ + 256 - 4;
+        LONG(0)
+    } > BOOT2_TEXT = 0x0
+
+    /* The second stage will always enter the image at the start of .text.
+       The debugger will use the ELF entry point, which is the _entry_point
+       symbol if present, otherwise defaults to start of .text.
+       This can be used to transfer control back to the bootrom on debugger
+       launches only, to perform proper flash setup.
+    */
+}
+
+
 INCLUDE "targets/arm.ld"