make attribute CFLAGS and LDFLAGS optional and add default empty value

Actually CFLAGS and LDFLAGS are not required and can be empty.
Without default empty value if target platform in project settings was
set to "Linux" CFLAGS and LDFLAGS was initialized with NoneType.
The result was broken build and it wasn't to save/load project with
such settings.

Traceback (most recent call last):
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 956, in _Build
if not builder.build() :
File "/home/developer/WorkData/PLC/beremiz/beremiz/targets/toolchain_gcc.py", line 96, in build
Builder_CFLAGS = ' '.join(self.getBuilderCFLAGS())
TypeError: sequence item 0: expected string, NoneType found

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Project overview

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In order to target the widest possible range of programmable devices and keep efficient, Beremiz use C code as an intermediate language. 

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To be executed, C needs to be compiled. `GCC <http://gcc.gnu.org>`_ serve that purpose perfectly.

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PLC program is expressed in languages defined in IEC-61131, including graphical languages. Thanks to PLCopen TC2, those graphical languages have a standardised representation, in XML.

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To be continued.