The Yocto team has requested that we do not use Poetry from within the
Makefile, as Yocto does not have network access during the build
process.
We want to maintain the current behaviour, so this change makes our use
of Poetry contigent on it being available in the environment.
Additionally, explicitly passing an empty toolchain parameter now allows
a tool to be *disabled* (e.g. passing `POETRY=` will prevent the build
system from trying to use Poetry).
Change-Id: Ibf552a3fee1eaadee767a1b948b559700083b401
Signed-off-by: Chris Kay <chris.kay@arm.com>
d286739716