configs/releng/syslinux/archiso_pxe-linux.cfg: replace checksum and verify with cms_verify=y
Specify cms_verify=y in SYSLINUX/PXELINUX configuration to use OpenSSL
CMS based method for verifying the root file system image against the
code signing certificates in the initramfs.
checksum and verify are removed since they essentially serve the same
purpose and performing all the checks just needlessly delays boot.
Additionally, the removal of verify allows to build the ISO without gpg,
i.e. without using mkarchiso's -g and -G options.
Fixes #200 (closed)
This also matches the proposed archlinux.ipxe change.