fs: Make efivarfs a pseudo filesystem, built by default with EFI

efivars is currently enabled under MISC_FILESYSTEMS, which is decribed
as "such as filesystems that came from other operating systems".
In reality, it is a pseudo filesystem, providing access to the kernel
UEFI variable interface.

Since this is the preferred interface for accessing UEFI variables, over
the legacy efivars interface, also build it by default as a module if
CONFIG_EFI.

Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>