efibootmgr: bump version

efibootmgr: bump version

Drop 0001-dont-use-fshort-wchar-when-building.patch and
0003-Remove-extra-const-keywords-gcc-7-gripes-about.patch as they are now
upstream.

The upstream repo moved to the 'rhboot' github project, so adjust upstream
URL in .mk and help text to match.

Version 15 introduces build time configuration of the default EFI directory
(E.G. the subdirectory in the EFI system partition where the loader is
installed).  This used to be hardcoded to redhat, but now a value must be
specified at build time.  Given that, it is unlikely that people relied on
the default value so set it to the more sensible 'buildroot'.

While we are at it, also add a hash for the license file.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>