targets.mk: fix inconsistent env variable BROKEN (#3103)

BROKEN TARGETS were built when BROKEN was set as a variable regardless of
it's value. Even an empty BROKEN variable caused BROKEN TARGETS to be
built. For DEVICES the number was evaluated and compared to true by Lua.

Make behavior consistent for relevant values UNSET/EMPTY/0 and 1:
UNSET/EMPTY/0: Don't build BROKEN TARGETS and DEVICES
1: Build BROKEN TARGETS and DEVICES

The behavior remains inconsistent for all positive numbers except 1.
(even floating point)

(cherry picked from commit bc8778c)

Co-authored-by: Felix Baumann <felix.bau@gmx.de>

targets/targets.mk

@@ -23,7 +23,7 @@ $(eval $(call GluonTarget,x86,legacy))
 $(eval $(call GluonTarget,x86,64))
 
 
-ifneq ($(BROKEN),)
+ifeq ($(BROKEN),1)
 $(eval $(call GluonTarget,bcm27xx,bcm2710)) # BROKEN: Untested
 $(eval $(call GluonTarget,mvebu,cortexa9)) # BROKEN: No 11s support
 endif