clr-boot-manager 1.5.5: Stability fix + vendor enabling

• Fix various build warnings
• Support vendor cmdline files
• Address upstream util-linux (libblkid) regression for LVM installations

Vendor fragments:

As of clr-boot-manager 1.5.5, it is now possible to ship fragments for the
final kernel command line (/proc/cmdline) entries in /usr/share/kernel/cmdline.d/*.conf

This allows vendors to ship quirks and behavioural changes via the cmdline in
a stateless fashion.

clr-boot-manager already had support for /etc/kernel/cmdline and /etc/kernel/cmdline.d/*.conf
fragments. This behaviour has been updated to support masking vendor entries by using the same
name for the /etc snippet as the one found in the vendor directory.

Additionally vendor fragments may be disabled entirely through this masking functionalty, by
symlinking the name to /dev/null.