Adding premake4.lua, minor adjustments

[assarbad]

• The premake4.lua was superficially tested for now
• Spelling fix in tomcrypt_cfg.h
• Using my changes to libtommath as template to fix up rng_get_bytes.c

This is just a draft at this point, I'll further hone this. But this way people can see and use it in its preliminary state.

NB: companion issue to this one: libtommath#525