binutils: bump 2.28.x series to 2.28.1

binutils 2.28.1 is a bug-fix release of the 2.28.x branch, so let's
switch to it.

Since a xz-compressed tarball is available, we use it instead of the
bz2-compressed one.

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