Add 'seoul_libc_support' library

This library contains the subset of the libc that is needed for the
Seoul VMM.

Related to issue #666.
Extend seoul_libc_support with string functions (currently) needed by seoul. Related to issue #666.
Switch to Seoul VMM repo instead of NUL.

In this version of the transition the Hip structure from Genode is reused, @nfeskes seoul_libc_support is used for the string functions and the nul/config.h is replaced by just using a constant value in the one place where the file was needed.

Related to #666.