Makefile tweaks

Hi,

this small addition allows me to use this options in Makefile.config:

CROSS_COMPILE = /opt/devel/nxp/toolchain/gcc-arm-none-eabi-4_7-2013q1/bin/arm-none-eabi-
JLINK_EXE = /opt/devel/jtag/segger-jlink/JLink_Linux_V485d_x86_64/StartJLinkExe.sh

As I usually prefer to explicitly select tools of my choice. For the JLinkExe commandline option, nrftool [Add jlinkexe commandline option](567ef6d43630202b5d1d282e0da76dc911ea987e) addition is needed.

Thanks.