Fix Makefile for boot/base having switched from maven to lein

[RadicalZephyr]

Fixes the problems with the Makefile targets clean and install.

Description

Currently the makefile has not been updated to reflect the fact that boot/base has been migrated to be built using lein rather than maven. As a result it errors when invoked with either of these tasks. This PR fixes that by replacing the mvn invocations with equivalent leininvocations.

How Has This Been Tested?

I ran make clean and make install and saw that they both completed successfully.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.