Makefile refactoring to minimize the number of targets

[shmuelk]

The Makefile has been refactored to:
1. Reduce the number of targets in the file by the use of targets and actions that exploit the % wildcard
2. Improved consistency in naming of targets.

In addition the following targets have changed:

• build now builds both of the EPP and the Sidecar simple binaries
• image-build now builds both of the EPP and the Sidecar images
• image-push now pushes both of the EPP and the Sidecar images
• test-unit now runs the unit tests of both the EPP and the Sidecar

To run the above targets for just one of the EPP or the sidecar, run:

• epp-buiild or sidecar-build
• epp-image-build or sidecar-image-build
• epp-image-push or sidecar-image-push
• epp-test-unit or sidecar-test-unit

Fixes: #385

[elevran]

/lgtm

[elevran]

@shmuelk please resolve conflict

[elevran]

/approve