Improve fix wrong code generation if EN/ENO are used in FBD/LD/SFC

Commit c9915bc6 "Fix wrong code generation if EN/ENO are used in
FBD/LD/SFC" caused following backtrace in some cases:

File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 476, in GenerateProgram
self.GeneratePouProgram(pou_name)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 258, in GeneratePouProgram
program = pou_program.GenerateProgram(pou)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 1713, in GenerateProgram
self.ComputeProgram(pou)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 997, in ComputeProgram
eno_var = self.GetUsedEno(body, connections)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 929, in GetUsedEno
for invar in blk.inputVariables.getvariable():
File "/home/developer/WorkData/PLC/beremiz/beremiz/xmlclass/xmlclass.py", line 1363, in getattrMethod
return DefaultElementClass.__getattribute__(self, name)
<type 'exceptions.AttributeError'>: 'inOutVariable' object has no attribute 'inputVariables'

all:
	@echo "Please specify target 'source_list', 'template' or 'locales'"

template:
	@echo To generate translation template message.pot file:
	python mki18n.py -p --domain=Beremiz

locales:
	@echo "Generate .mo files for all languages:"
	python mki18n.py -m --moTarget=../locale --domain=Beremiz


source_list:
	echo @To "Generate list with source files: app.fil:"
	find .. -name "*.py" -exec grep -q '_(' {} ';' -print -o -name "*XSD*" -print -o -name "*.csv" -print | grep -v '/build/' | grep -v '/pyjs/' > app.fil
	echo "../plcopen/Additional_Function_Blocks.xml" >> app.fil
	echo "../plcopen/Standard_Function_Blocks.xml"   >> app.fil

.PHONY: all, template, locales, source_list