config: add KERNEL_LSM symbol

The LSM (Linux security mechanism) list is the successor of the now
legacy *major LSM*. Instead of defining a single security mechanism the
LSM symbol is a comma separated list of mechanisms to load.

Until recently OpenWrt would only support DAC (Unix discretionary access
controls) which don't require an additional entry in the LSM list. With
the newly introduced SELinux support the LSM needs to be extended else
only a manual modified Kernel cmdline (`security=selinux`) would
activate SELinux.

As the default OpenWrt Kernel config sets DAC as default security
mechanism, SELinux is stripped from the LSM list, even if
`KERNEL_DEFAULT_SECURITY_SELINUX` is activated. To allow SELinux without
a modified cmdline this commit sets a specific LSM list if
`KERNEL_SECURITY_SELINUX` is enabled.

The upstream Kconfig adds even more mechanisms
(smack,selinux,tomoyo,apparmor), but until they're ported to OpenWrt,
these can be ignored.

To compile SELinux Kernel support but disable it from loading, the
already present options `KERNEL_SECURITY_SELINUX_DISABLE` or
`KERNEL_SECURITY_SELINUX_BOOTPARAM` (with custom cmdline `selinux=0`)
can be used. Further it's possible to edit `/etc/selinux/config`.

Signed-off-by: Paul Spooren <mail@aparcar.org>

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