make: rename CFLAGS in target Makefiles to TGT_CFLAGS

Renamed every instance of variable CFLAGS in target specific Makefiles
to TGT_CFLAGS to free up CFLAGS for user defined compiler flags.

Added information in README.md about existence and usage of CFLAGS
environment variable in build process.

lib/Makefile.include

@@ -42,7 +42,7 @@ $(SRCLIBDIR)/$(LIBNAME).ld: $(LIBNAME).ld
 
 %.o: %.c
 	@printf "  CC      $(<F)\n"
-	$(Q)$(CC) $(CFLAGS) -o $@ -c $<
+	$(Q)$(CC) $(TGT_CFLAGS) $(CFLAGS) -o $@ -c $<
 
 clean:
 	$(Q)rm -f *.o *.d ../*.o ../*.d