libports: add vfs_jitterentropy library

This file system library uses the the jitterentropy library to provide
a rudimentary '/dev/random' device.

Fixes #1239.
libports: add jitterentropy-20140411

This commit adds a port the jitterentropy library to Genode. As
backend on x86_{32,64} 'rdtsc' is used and on ARMv{6,7} the
performance-counter.

Fixes #1239.