Fix default target: exploded-image

[keithc-ca]

Adjust build rules to satisfy expectations of exploded-image make target.

• don't use 'make -C' unnecessarily
• don't nest shells unnecessarily
• use consistent make command sequence
• move dependencies to closed/make/Main.gmk

Fixes #34

[pshipton]

Does the ARM build still work with these changes?

jenkins compile all

[keithc-ca]

I expect the ARM build to continue to work. I need to reconnect with @JamesKingdon to get current configuration instructions.

[pshipton]

No Windows machines are available yet.

[keithc-ca]

I revived a docker container that I had previously used to create an ARM build. Updating the repos to include this change it still builds fine (and supports exploded-image too, of course).

[JamesKingdon]

Thanks Keith, much appreciated.