wireless-regdb: fix build when python2 from package feeds exists

wireless-regdb: fix build when python2 from package feeds exists

wireless-regdb fails to build if there is python2 installed from package
feeds, as staging_dir/hostpkg/bin/python is python2 and
staging_dir/hostpkg/bin takes precedence over staging_dir/host/bin
(proper place with python -> python3 symlink) which leads to the build
failure of wireless-regdb, so this patch makes it explicit which python
should be used.

Reported-by: Hauke Mehrtens <[email protected]>
Tested-by: Kevin Darbyshire-Bryant <[email protected]>
Tested-by: Russell Senior <[email protected]>
Tested-by: Lucian Cristian <[email protected]>
Signed-off-by: Petr Štetiar <[email protected]>