Probably it should be:
$this->bbcode_second_pass_code('', 'rm "${pkgdir}"/boot/{start*x.elf,fixup*x.dat}')
$this->bbcode_second_pass_code('', 'cp "${srcdir}"/firmware-${_commit}/boot/{start*x.elf,fixup*x.dat} "${pkgdir}"/boot')
in the raspberrypi-bootloader PKGBUILD instead?