efivar: disable for some incompatible Sourcery CodeBench toolchains

efivar: disable for some incompatible Sourcery CodeBench toolchains

linux/nvme.h has been renamed to linux/nvme_ioctl.h since 4.4, and we
have a patch to handle that situation. However, some Sourcery CodeBench
toolchains use 4.4 kernel headers but don't have that header file
renamed, and that is causing build failures. So, prevent this package
to be built using those toolchains.

Fixes:

  http://autobuild.buildroot.net/results/66c/66cf68261fa73c2366610f8d14bd02180ba7cd2d/
  http://autobuild.buildroot.net/results/54c/54cc7d4e3b284b35931e08f9c7f0f902df96a989/

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
[Romain:
  - add back slash on line breaks
  - propagate efivar dependencies to efibootmgr
  - add comments dependencies]
Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>