Extend python<->PLC global variable description "GlobalDesc" to support Initial value setting.

Rebased from dporopat change "add initial value to runtime.py file", which means :
- GlobalDesc is used by some customized beremiz python runtime
- Those runtimes need to know what is initial value of these special variables

BEREMIZ_OBJS = $(BEREMIZSRC:.c=.o)

all: warn some_binary
	@echo "*** all done ***"

warn:
	@echo "*** Sample Makefile, does nothing ***"

some_binary: $(BEREMIZ_OBJS)
	@echo "* Would link $^ -> $@"

%.o: %.c
	@echo "* Would compile $< -> $@"