NuttX build in place

- cmake NuttX build wrapper compile in place instead of copying source tree to build directory
    - slightly faster skipping necessary copying (depending on system)
    - allows debugging in place
    - easier to work directly in NuttX following official documentation
    - simplifies overall build which should make it easier to resolve any remaining NuttX dependency issues in the build system
 - the downside is switching back and forth between different builds always require rebuilding NuttX, but I think this is worth the improved developer experience
 - also no longer builds px4io and bootloader in every single build, for most users these rarely change and we're wasting a lot of build time

platforms/nuttx/init/rc.board_bootloader_upgrade.in

@@ -0,0 +1,17 @@
+#! /bin/sh
+
+#
+# Bootloader upgrade
+#
+if param compare -s SYS_BL_UPDATE 1
+then
+	if [ -f "/etc/extras/@BOARD_FIRMWARE_BIN@" ]
+	then
+		param set SYS_BL_UPDATE 0
+		param save
+		echo "bootloader update..."
+		bl_update "/etc/extras/@BOARD_FIRMWARE_BIN@"
+		echo "bootloader update done, rebooting"
+		reboot
+	fi
+fi