Have syslinux' SYSAPPEND feature add FSUUID= to the kernel command-line.

This will help addressing #6397, without having to hard-code in the bootloader
configuration the UUID of the FS the SquashFS should be loaded from.
Install syslinux-efi: since syslinux 3:6.03~pre1+dfsg-2, that's where the EFI bootloader lives.
Adapt syslinux EFI bootloader path to cope with changes in the Debian package.
Bump pining value for wheezy-backports' shared-mime-info.

The idea is to override the custom 1.2 + upstream cherry-pick backport
we have in our APT repository. This commit shall be reverted when merging this
branch and removing that custom package.
Merge branch 'bugfix/7079-shared-mime-info-1.3' into experimental
Merge branch 'feature/6397-stop-relying-on-the-removable-bit' into experimental