base/bitbake.conf: Filter contents of PATH to only allow whitelisted tools

base/bitbake.conf: Filter contents of PATH to only allow whitelisted tools

We currently have a determinism problem in that the host tools present
in PATH can influence the build. In particular, the presence of pkg-config
on the build host can mask missing pkgconfig class dependencies.

This adds in a new HOSTTOOLS variable and then uses it to set up a directory
of symlinks to the whitelisted host tools. This directory is placed as PATH
instead of the usual /usr/bin:/bin and so on.

This should improve determinism of builds and avoid the issues which have
been particularly obvious since the introduction of recipe specific sysroots.

If users find there is a tool missing, they can extend HOSTTOOLS from a global
class or global conf file.

Right now the settings should be enough to build everything in OE-Core.

(From OE-Core rev: fa764a403da34bb0ca9fa3767a9e9dba8d685965)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>