- Longer explanation of multi-BR2_EXTERNAL
- Mention nobody UID 99 -> 65534 change
- Mention removal of BR2_DEPRECATED
- Mention use of system cmake
- Mention new dependency on ExtUtils::MakeMaker. The other perl
  dependencies are not mentioned since they only apply if mpv is built.
- Mention the $(STAGING_DIR)/$(O) check.
- Mention unsafe path detection in -isystem etc.
- Mention linux-tools split.
- Mention changes in legal-info.
- Mention that pseudo is used instead of fakeroot.
- Correct the info of toolchainfile.cmake: it still sets cflags but it
  doesn't force them anymore (doesn't append).

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>