bootstrap: bump version to 4.0.0

other changes:
  - Add a license file to bootstrap.mk
  - Add a hash for the license file to bootstrap.hash
  - Remove copying bootstrap-theme.min.css as it's no longer included.
  - Remove copying fonts folder as it's no longer included.

Signed-off-by: Adam Duskett <Adamduskett@outlook.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>