efivar: bump to version 28

efivar: bump to version 28

Upstream has changed version numbering scheme since version 25, which
came after version 0.24.

The two patches that we had in package/efivar/ were backports and so can
be dropped with this bump.

This new version also fixes the build failure with gcc 6.x, so:
Fixes:
http://autobuild.buildroot.net/results/ea6/ea6968d5d0fd0d8ee64e045e2aa56c8e8a0b7b14

[Peter: require GCC 4.9+, adjust Config.in nvme.h comment]
Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>