The pacman.conf shipped in pacman 5.1.0 now has package signing enabled by default. After upgrading to this version, if you haven't already enabled package signing and imported our signing keys you will need to do that before installing further packages.
To initialize your pacman keyring and import the package signing keys:
$this->bbcode_second_pass_code('', 'pacman-key --init && pacman-key --populate archlinuxarm')
For additional information on our package signing, go here.