Pulled vga.h out from kernel.c

Now that I got my environment set up, I'm trying to follow along with
the meatier skeleton on wiki.osdev.org without copy pasting everything
this time.
Removed bool stuff

Here I was thinking that it might not be necessary with c11 - but we're
not even using them.
Changed vga memory pointer into a named constant

As opposed to a magic constant and following along the meatier skeleton.
In my opinion, this is one of the more interesting aspects of OS (and
driver) development. In application development, something like this -
taking a non-zero integer literal and turning it into a pointer would
surely be wrong. In OS development, though, this is correct (if not very
resilient against memory corruption).
Moved boot.s and linker.ld to their new home

This is still following along the meatier skeleton at wiki.osdev.org.
Also tweaked the Tupfile to minimize code duplication.