waf: Improve version parsing to avoid failing on warnings

waf: Improve version parsing to avoid failing on warnings

waf uses an inline tar file extracted by the tarfile module. The tarfile
module may print a warning when used with default 'filter' argument[0].

When called to get the version, the first time after unpack, the output
may look like:
  # output from lower modules (e.g: warnings from tarfile, ...)
  waf X.Y.Z ...

This patch makes the version parsing more precise by looking at the
first line matching "waf ".

[0]: https://docs.python.org/3.12/library/tarfile.html#extraction-filters

Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>