Conda pack support

[PiotrZierhoffer]

This PR is dependent on https://github.com/timvideos/litex-buildenv/pull/212

It adds two scripts: build-pack.sh and enter-pack.sh that allow to create and use the package.

build-pack.sh can be started on a clean lxbe repo, building a package in ./build/lxbe-env.tar.gz. It weights about 1.3GB.

The pack contains a Conda environment, which is subsequently used by enter-pack.

enter-pack.sh must be sourced and requires the usual variables to be set (e.g. export TARGET=base FIRMWARE=zephyr PLATFORM=arty CPU_VARIANT=full CPU=vexriscv). It unpacks the provided tar.gz (or uses the already unpacked directory) and activates the environment.

This is an important change to the way enter-env.sh operates (that is - using the default env, without activation).

After the activation, the usual tools can be used to build/run/flash things.

[todo[bot]]

requirements.txt?

https://github.com/timvideos/litex-buildenv/blob/9d34cbb2ec7264a88e29919803c31c257140d58c/scripts/build-pack.sh#L109-L114

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This comment was generated by todo based on a TODO comment in 9d34cbb2ec7264a88e29919803c31c257140d58c in #213. cc @antmicro.

[mithro]

Please rebase onto master.

[mithro]

@mateusz-holenko -- Could you work with @PiotrZierhoffer to get something like this pull request merged?