efi_loader: move efi_var_collect to common functions

efi_var_collect() was initially placed in efi_var_file.c, since back
then we only supported efi variables stored in a file. Since then we
support variables stored in an RPMB as well and use that function to
collect variables that should be present at runtime.

So let's move it around in efi_var_common.c which makes more sense

Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Ilias Apalodimas <apalos@gmail.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>