add amc-update-db: fixes error in devstack make dev.reset

[OmarIthawi]

More fixes for make dev.reset RED-2047.

This replaces the generic %-update-db Makefile rule which fails for AMC with an AMC-specific one that works.

docker-compose -f docker-compose.yml -f docker-compose-host.yml -f docker-compose-themes.yml -f docker-compose-watchers.yml -f docker-compose-xqueue.yml -f docker-compose-analytics-pipeline.yml -f docker-compose-marketing-site.yml exec amc bash -c 'source /edx/app/amc/amc_env && cd /edx/app/amc/amc/ && make migrate'
[sudo] password for omar: 
bash: /edx/app/amc/amc_env: No such file or directory
make: *** [Makefile:316: amc-update-db] Error 1