Merge #31

31: New linker script and multi-core support r=laanwj a=Disasm

* Linker script was reworked. Now it uses region aliases to relocate sections. This approach makes it possible to build firmware for both FLASH+RAM and RAM-only targets. Memory definitions now supposed to be present in their corresponding crates (e.g. RAM definition in PAC crate, FLASH definition in board support crate).
* Multi-core support was introduced. Cores are parked with `_mp_hook` function and then awakened in platform-dependent way.
* Documentation was updated to reflect new features.
* New crate version: 0.6.0

Depends on: https://github.com/rust-embedded/riscv/pull/28
Closes: https://github.com/rust-embedded/riscv-rt/issues/26



Co-authored-by: Vadim Kaushan <admin@disasm.info>