package/libselinux: set the config_lsm kernel config option to selinux

Currently, the libselinux package sets the CONFIG_DEFAULT_SECURITY_SELINUX
kernel option. However, as of kernels >= 5.1, this option is superseded in
favor of the CONFIG_LSM option, a comma-separated list of LSMs the kernel
should initialize in order.

As the previous behavior of this package sets the kernel's default and only
LSM to initialize to SELinux, it is safe to set this string to just selinux.
If the user wants additional LSM's, they may do so with a custom kernel config.

Signed-off-by: Adam Duskett <Aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>