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.

@aabadie

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

(merged)

@fjmolinas

[#14351] Makefile.include: resolve dependencies before Makefile.include

(merged)

@kaspar030

[#14224] core: make idle thread optional

(merged)

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

(postponed to 2020.10)

@miri64

[#12907] tinydtls: add sock_async support for sock_dtls

(merged)

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

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

(merged)

[#14349] pkg/lwip: enable IPv4/IPv6 dual stack mode

(merged)

[#12647] paho-mqtt: add package and example running on native board