trezor: hide behind compile tag

The trezor libraries are not yet stable enough to build
gocryptfs with trezor support by default.

It does not even compile at the moment:

  $ ./build.bash -tags enable_trezor
  # github.com/conejoninja/tesoro/vendor/github.com/trezor/usbhid
  ../../conejoninja/tesoro/vendor/github.com/trezor/usbhid/hid.go:32:11: fatal error: os/threads_posix.c: No such file or directory
    #include "os/threads_posix.c"
           ^~~~~~~~~~~~~~~~~~~~
  compilation terminated.

https://github.com/conejoninja/tesoro/issues/9

internal/readpassword/trezor.go

@@ -1,3 +1,5 @@
+// +build enable_trezor
+
 package readpassword
 
 import (

internal/readpassword/trezor_disabled.go

@@ -0,0 +1,19 @@
+// +build !enable_trezor
+
+package readpassword
+
+import (
+	"os"
+
+	"github.com/rfjakob/gocryptfs/internal/tlog"
+)
+
+const (
+	TrezorPayloadLen = 32
+)
+
+func Trezor(payload []byte) []byte {
+	tlog.Fatal.Printf("\"-trezor\" is not implemented yet.")
+	os.Exit(1)
+	return nil
+}