efivar: depends on target gcc >= 4.7

efivar: depends on target gcc >= 4.7

The efivar internal build system uses flags such as -std=gnu11 ,
-Wmaybe-uninitialized , -flto which are available in gcc >= 4.7 only.
These flags are passed both to target build and to build a host tool.
For the host part, this has been worked around by overriding
'gcc_flags', but doing that for the target would remove all flags that
upstream intended for the target build.

Buildroot doesn't support building gcc 4.6 in its own toolchain anymore
but it's possible to use an external toolchain with gcc 4.6 which would
be unable to build this package.

This patch adds a limitation on toolchains with target gcc >= 4.7 to
make sure that the flags are available in the chosen toolchain.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>