fix makefile templates for nrf51822 targets

+ disable optimization which could make a program dead
+ use `%.o: %.c` instead of `.c.o` which is an old-fashioned suffix rule and doesn't detect source files' changes.
+ don't merge hex files by default to avoid error message when srec_cat is not available. to merge hex files, run `make merge`. the merged hex file seems not work (at least for Arch BLE)