Make the mode configurable at build time for the waltham transmitter

Make the mode configurable at build time for the waltham transmitter

- Create a TRANSMITTER_OUTPUT_MODE variable with a known default
- Use this to update the transmitter-output.cfg fragment
- Users can redefine this variable based on their needs
- While at it do the same for HOST and PORT

Bug-AGL: SPEC-4030

Signed-off-by: Anusha Gugale <external.agogale@jp.adit-jv.com>
Signed-off-by: Jan-Simon Möller <jsmoeller@linuxfoundation.org>
Change-Id: I32374186bdce3171a1f7ad4026e10c6448edaec6
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/26609
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account