add rp2040, pico

adds preliminary support (just enough to run blinky1) for the Raspberry Pi Pico board along with the rp2040 mcu.

targets/pico.ld

@@ -0,0 +1,31 @@
+
+MEMORY
+{
+    FLASH_TEXT (rx) : ORIGIN = 0x10000000, LENGTH = 2048k
+}
+
+SECTIONS
+{
+    /* Second stage bootloader is prepended to the image. It must be 256 bytes big
+       and checksummed. It is usually built by the boot_stage2 target
+       in the Raspberry Pi Pico SDK
+    */
+
+    .boot2 : {
+        __boot2_start__ = .;
+        KEEP (*(.boot2))
+        __boot2_end__ = .;
+    } > FLASH_TEXT
+
+    ASSERT(__boot2_end__ - __boot2_start__ == 256,
+        "ERROR: Pico second stage bootloader must be 256 bytes in size")
+
+    /* 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/rp2040.ld"