High impact PRs for 2020.07 release

These PRs must be merged before 2020-06-26 or won't be in the 2020.07 release:

[#12457] cpu/msp430: update to modern gcc/ELF/newlib toolchain

(postponed to 2020.10)

[#14224] core: make idle thread optional

(merged)

[#14289] pkg: store packages sources in a global package directory

(merged)