v1.2.0: Configurable & more robust clr-boot-manager

• Swap the in-tree nica for a git submodule, easing maintenance.
• Remove hardcoding of names:
  • Pull the OS title from the os-release file (PRETTY_NAME)
  • Take the "vendor-prefix" as a configure option.
• Change update mechanism to always attempt update, and only update what needs changing. This allows instant configuration changes.
• Add support for command line appending from /etc/kernel/cmdline and /etc/kernel/cmdline.d/*.conf. These files may contain comments (#) and newlines, and will be cleaned automatically before appending.
• Enhanced timeout behaviour:
  • Alter set-timeout and get-timeout to use the plain text /etc/kernel/timeout file.
  • Timeout change can be achieved by just running clr-boot-manager update now, as we always attempt updating files if they changed.
• Enforce C11 in the codebase

Example configuration for UEFI distributions:

./configure --prefix=/usr --libdir=/usr/lib64 --sysconfdir=/etc --with-vendor-prefix=SomeOS --with-kernel-modules-dir=/lib/modules --with-kernel-namespace=org.someos --with-boot-dir=/boot

Version 2.x will feature more changes to enable initrd, GRUB, and automatic LVM on LUKS support in kernel cmdline.