Fix classpath_cache bug

[jerryz123]

Previously, the chipyard.jar cache was built with SBT_PROJECT (usually chipyard) as the top-level project. This causes a bug where after building the jar with SBT_PROJECT=chipyard, future fpga compilations (with SBT_PROJECT=fpga_platforms) would fail due to not rebuilding the cache.

This PR updates the build system to build a separate jar for the fpga flow that includes the fpga-specific sources.

This renames fpga_platforms to chipyard_fpga to make it clearer that that project is an extension of chipyard with fpga configs.

This renames CHIPYARD_CLASSPATH to GENERATOR_CLASSPATH to make it clearer that the this classpath must contain a Generator main to run, but it does not have to be the chipyard project in build.sbt

Related PRs / Issues:

Type of change:

• Bug fix
• New feature
• Other enhancement

Impact:

• RTL change
• Software change (RISC-V software)
• Build system change
• Other

Contributor Checklist:

• Did you set main as the base branch?
• Is this PR's title suitable for inclusion in the changelog and have you added a changelog:<topic> label?
• Did you state the type-of-change/impact?
• Did you delete any extraneous prints/debugging code?
• Did you mark the PR with a changelog: label?
• (If applicable) Did you update the conda .conda-lock.yml file if you updated the conda requirements file?
• (If applicable) Did you add documentation for the feature?
• (If applicable) Did you add a test demonstrating the PR?

• (If applicable) Did you mark the PR as Please Backport?

[abejgonzalez]

LGTM