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

/* Beremiz' header file for use by extensions */

#include "iec_types.h"

#define LOG_LEVELS 4
#define LOG_CRITICAL 0
#define LOG_WARNING 1
#define LOG_INFO 2
#define LOG_DEBUG 3

extern unsigned long long common_ticktime__;
int LogMessage(uint8_t level, char* buf, uint32_t size);
long AtomicCompareExchange(long* atomicvar,long compared, long exchange);