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
By default, syslinux interprets TFTP paths as relative to the location of the *.c32 modules, regardless whether a path starts with a slash. Without the `::` that I added to all of these paths, syslinux cannot find these files. It searches for them in /%INSTALL_DIR%/syslinux//%INSTALL_DIR%/boot/x86_64/vmlinuz-linux, etc.
Slightly simplifies adding boot loader configuration for more kernels.
Unfortunately the INCLUDE statement doesn't support wildcards, so each new file must be manually included in the main syslinux configuration file (syslinux.cfg for baseline and archiso_sys.cfg for releng).
