targets: add USES_PM auto-feature

Not all targets support power management, some older or more simple
targets don't have CONFIG_PM set. Allow kernel module packages to
depend on USES_PM to only be available on targets which got
CONFIG_PM=y in their kernel config.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

include/target.mk

@@ -318,6 +318,9 @@ ifeq ($(DUMP),1)
     ifneq ($(CONFIG_PINCTRL),)
       FEATURES += pinctrl
     endif
+    ifneq ($(CONFIG_PM),)
+      FEATURES += pm
+    endif
     ifneq ($(CONFIG_PWM),)
       FEATURES += pwm
     endif