usbboot: conditionally sign msd binaries

With the USB provisioning being preferred, the msd binaries are no
longer required.

This commit adds a recipe configuration to make the signing conditional.

Changelog-entry: do not sign msd binaries by default as they are not needed
Signed-off-by: Alex Gonzalez <alexg@balena.io>

layers/meta-balena-raspberrypi/recipes-devtools/usbboot/usbboot-native_git.bb

@@ -20,7 +20,7 @@ do_install(){
   install -m 644 ${S}/msd/bootcode4.bin ${D}
   install -m 644 ${S}/msd/start.elf ${D}
   install -m 644 ${S}/msd/start4.elf ${D}
-  if [ "x${SIGN_API}" != "x" ]; then
+  if [ "x${SIGN_API}" != "x" ] && [ "${BALENA_SIGN_MSD}" = "1" ]; then
      install -d ${D}/secure-boot-msd/
      if ! do_sign_rsa "${S}/secure-boot-msd/boot.img" "${D}/secure-boot-msd/boot.sig"; then
         bbfatal "Failed to sign boot image"