go-bootstrap: new host package

Add a new package go-bootstrap which builds a bootstrap compiler for the
go programming language.

Signed-off-by: Geoff Levand <geoff@infradead.org>
[Thomas:
 - Rename GO_BOOTSTRAP_FINAL to HOST_GO_BOOTSTRAP_ROOT. Use the HOST_
   prefix because this variable is specific to the host variant. And
   it's not "final" at all, but rather the "root" of the Go Bootstrap
   installation.
 - Rename GO_BOOTSTRAP_MAKE_ENV to HOST_GO_BOOTSTRAP_MAKE_ENV, also to
   have the HOST_ prefix because it's a host package.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>