Add Functionality to Primary Makefile

[blazebissar]

Currently there are two makefiles: one for only toynet-flask and one for only toynet_mininet that perform similar functions for each docker image.

Update the toynet-flask/Makefile so that, in addition to its current functionality, it can call the toynet-flask/toynet_mininet/Makefile functionality as well. If it is possible, also update it to conditionally build the toynet-flask/toynet_mininet/*.Dockerfile files when those images require it when running the test suites.

We should also add a make lint capability.

[ajlacey]

Notes:
https://www.gnu.org/software/make/manual/make.html#Recursion

make lint should be integrated into another feature that will call all of the tests. Proposed name: make pr-validate