x86: generic: disable CONFIG_EFI_VARS

x86: generic: disable CONFIG_EFI_VARS

CONFIG_EFI_VARS has been disabled in
64bit x86 target in 2018 by the following commit
b0a51dab8c5b8d40828a00b0c6d66450bf90f217
the same reasons apply to Generic target, so
now it's disabled here too.
Leaving it enabled is also blocking compile as
a new symbol was added
EFI_CUSTOM_SSDT_OVERLAYS
that depends from CONFIG_EFI_VARS
and the build system stops and waits for
user input on what to do about it.

The Legacy and Geode targets never
had any EFI_xxx configs enabled so they
don't have this issue

Signed-off-by: Alberto Bursi <bobafetthotmail@gmail.com>