powerpc: add security.config, enforcing lockdown=integrity

It's sometimes handy to have a config that boots a bit like a system
under secure boot (forcing lockdown=integrity, without needing any
extra stuff like a command line option).

This config file allows that, and also turns on a few assorted security
and hardening options for good measure.
Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201203042807.1293655-1-dja@axtens.net

arch/powerpc/configs/security.config

+# This is the equivalent of booting with lockdown=integrity
+CONFIG_SECURITY=y
+CONFIG_SECURITYFS=y
+CONFIG_SECURITY_LOCKDOWN_LSM=y
+CONFIG_SECURITY_LOCKDOWN_LSM_EARLY=y
+CONFIG_LOCK_DOWN_KERNEL_FORCE_INTEGRITY=y
+
+# These are some general, reasonably inexpensive hardening options
+CONFIG_HARDENED_USERCOPY=y
+CONFIG_FORTIFY_SOURCE=y
+CONFIG_INIT_ON_ALLOC_DEFAULT_ON=y
+
+# UBSAN bounds checking is very cheap and good for hardening
+CONFIG_UBSAN=y
+# CONFIG_UBSAN_MISC is not set
\ No newline at end of file