High priority PRs for 2020.07 release

This is a list of PRs, that a contributor has identified as a high priority to them for the 2020.07 release.

@kaspar030

[#14224] core: make idle thread optional

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

@miri64

[#12907] tinydtls: add sock_async support for sock_dtls

[#12975] posix: initial import of select() function (only support sockets for now)

[#13221] dist/tools: provide tool to parse pktbuf output