fix nrfjprog file not found on windows issue #639

Author: hathach

programmers.txt

@@ -3,8 +3,8 @@
 #**********************************************
 nrfjprog.name=J-Link for Bluefruit nRF52
 nrfjprog.program.tool=bootburn
-nrfjprog.program.cmd=nrfjprog
-nrfjprog.program.cmd.windows=nrfjprog.exe
+nrfjprog.program.cmd={runtime.tools.nrfjprog.path}/nrfjprog
+nrfjprog.program.cmd.windows={runtime.tools.nrfjprog.path}/nrfjprog.exe
 
 # Burn bootloader pattern
 nrfjprog.program.burn_pattern="{program.cmd}" --program "{bootloader.file}.hex" -f nrf52 --chiperase --reset