PACKER_PATH arg for Makefile to fix support packer symlinks and/or conflicting paths.

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#### Description

On some distributions, there may be another tool named packer installed by default. This prevents using the Makefile to build when this conflict is present.
Additionally packer recommends using a symlink to overcome this conflict. Using the PACKER_PATH arg, it could be set to the symlink that points to packer as well.

See description of issue in Packer documentation listed below in references.

#### Use Case(s)

When on a distribution of linux and doing development on the packer-plugin-amazon and the distribution has the cracklib packer conflict, users will have other either run the make dev target commands by hand or edit the makefile to use the absolute path of packer.

Instead an optional arg 'PACKER_PATH' that defaults to 'packer' will allow users to optionally change the packer to use.

#### Potential configuration
Using the absolute path to packer
```
make PACKER_PATH=/usr/local/bin/packer dev
```

Using a symlink to packer named packer.io that exists in users home directory
```
make PACKER_PATH=~/packer.io dev
```
#### Potential References
https://developer.hashicorp.com/packer/tutorials/docker-get-started/get-started-install-cli#troubleshooting