debian/rules.gen is not regenerated after architecture or build profile change

common.mk makes debian/rules.gen depend on debplate-config.yml and all .rules templates, but it is also affected by changes to the host architecture and build profiles. debplate should use a similar trick to Kbuild to make it depend on these:

• Add a phony dependency to it
• Skip all the commands if no prerequisites have changed, and the build environment hasn't changed (see if_changed and arg-check macros)