boot_serial: Update cddl-gen

Update and rename submodule.
Regenerate code and copy updated files.
Update regeneration script.

Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>
diff --git a/.gitmodules b/.gitmodules
index 1e4e333..adcdcb8 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -19,6 +19,6 @@
 [submodule "boot/cypress/libs/cy-mbedtls-acceleration"]
 	path = boot/cypress/libs/cy-mbedtls-acceleration
 	url = https://github.com/cypresssemiconductorco/cy-mbedtls-acceleration.git
-[submodule "boot/boot_serial/src/cddl_gen"]
-	path = ext/cddl_gen
-	url = https://github.com/oyvindronningstad/cddl_gen.git
+[submodule "ext/cddl-gen"]
+	path = ext/cddl-gen
+	url = https://github.com/NordicSemiconductor/cddl-gen.git