digsig: Remove MODULE check in dsi.h

Remove check for MODULE preprocessor definition in dsi.h, which is no longer
required because the code will not be running outside the kernel.

Signed-off-by: Rupert Muchembled <rupert@rmuch.net>
digsig: Integrate into Linux build system

Add Kconfig file and standard Makefile for DigSig, and integrate fully into
the kernel security code.

Signed-off-by: Rupert Muchembled <rupert@rmuch.net>