zephyr: prj.conf: disable CONFIG_BT_CTLR too

Various Zephyr boards (I'm looking at you, every-single-nRF-board)
set CONFIG_BT_CTLR=y as well as CONFIG_BT. Make sure it's disabled for
MCUboot's default prj.conf, where it's not needed and has been causing
very verbose Kconfiglib warnings:

    warning: BT_CTLR (defined at subsys/bluetooth/controller/Kconfig:10) was assigned the value 'y' but
    got the value 'n'. You can check symbol information (including dependencies) in the 'menuconfig'
    interface (see the Application Development Primer section of the manual), or in the Kconfig
    reference at http://docs.zephyrproject.org/reference/kconfig/CONFIG_BT_CTLR.html (which is updated
    regularly from the master branch). See the 'Setting configuration values' section of the Board
    Porting Guide as well.

Signed-off-by: Marti Bolivar <marti@foundries.io>
diff --git a/boot/zephyr/prj.conf b/boot/zephyr/prj.conf
index 60fd5e8..aec69e8 100644
--- a/boot/zephyr/prj.conf
+++ b/boot/zephyr/prj.conf
@@ -32,6 +32,7 @@
 
 CONFIG_MULTITHREADING=n
 
-### Zephyr keeps turning on major subsystems by default that we don't want.
+### Various Zephyr boards enable features that we don't want.
 # CONFIG_BT is not set
+# CONFIG_BT_CTLR is not set
 # CONFIG_I2C is not set