[packages][magma] Split magma-vsl-gc into a separate package

This way it can be included by default on most platforms (through the board .gni),
but won't be included on astro. That will save around 600KB uncompressed.

Test: CQ
Change-Id: Ibe4d8ba90a4ad563e086a976299a82eff5bf42e3
4 files changed