build: xtools: Honour CROSS_COMPILE (if set)
build: xtools: Honour CROSS_COMPILE (if set)

crosstool-ng toolchains are relocatable so, while the assumption it will
live in $(XTOOLS_TOOLCHAIN_PATH)/$(CROSS_COMPILE_TARGET) is a reasonable
default, it is useful to have to an quick and easy way to override that.

Reusing CROSS_COMPILE works well for this; it allows
ZEPHYR_GCC_VARIANT=xtools to set up the compiler and linker flags
without being too opinionated about anything else.

Change-Id: I6edd97a3753f2a917a34f723273416f76caa647a
Signed-off-by: Daniel Thompson <4421c302a8f2019ff02452a16ddbdd37770f814c@linaro.org>

runchip/zephyr-cc3220,galak/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,fbsder/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,pklazy/zephyr,kraj/zephyr,fractalclone/zephyr-riscv,runchip/zephyr-cc3200,runchip/zephyr-cc3220,runchip/zephyr-cc3200,tidyjiang8/zephyr-doc,sharronliu/zephyr,galak/zephyr,rsalveti/zephyr,bboozzoo/zephyr,finikorg/zephyr,fbsder/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,rsalveti/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,aceofall/zephyr-iotos,explora26/zephyr,galak/zephyr,Vudentz/zephyr,aceofall/zephyr-iotos,sharronliu/zephyr,finikorg/zephyr,bboozzoo/zephyr,mbolivar/zephyr,fractalclone/zephyr-riscv,fbsder/zephyr,Vudentz/zephyr,pklazy/zephyr,tidyjiang8/zephyr-doc,GiulianoFranchetto/zephyr,kraj/zephyr,erwango/zephyr,pklazy/zephyr,tidyjiang8/zephyr-doc,punitvara/zephyr,aceofall/zephyr-iotos,GiulianoFranchetto/zephyr,holtmann/zephyr,erwango/zephyr,zephyriot/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,pklazy/zephyr,holtmann/zephyr,bigdinotech/zephyr,ldts/zephyr,nashif/zephyr,bboozzoo/zephyr,explora26/zephyr,explora26/zephyr,fbsder/zephyr,rsalveti/zephyr,nashif/zephyr,bigdinotech/zephyr,rsalveti/zephyr,sharronliu/zephyr,bigdinotech/zephyr,ldts/zephyr,bboozzoo/zephyr,pklazy/zephyr,runchip/zephyr-cc3200,zephyriot/zephyr,nashif/zephyr,ldts/zephyr,rsalveti/zephyr,fractalclone/zephyr-riscv,runchip/zephyr-cc3220,zephyriot/zephyr,aceofall/zephyr-iotos,zephyrproject-rtos/zephyr,bigdinotech/zephyr,fractalclone/zephyr-riscv,punitvara/zephyr,nashif/zephyr,ldts/zephyr,galak/zephyr,erwango/zephyr,erwango/zephyr,fractalclone/zephyr-riscv,tidyjiang8/zephyr-doc,runchip/zephyr-cc3200,kraj/zephyr,ldts/zephyr,Vudentz/zephyr,Vudentz/zephyr,aceofall/zephyr-iotos,sharronliu/zephyr,holtmann/zephyr,runchip/zephyr-cc3220,bigdinotech/zephyr,holtmann/zephyr,runchip/zephyr-cc3220,sharronliu/zephyr,tidyjiang8/zephyr-doc,erwango/zephyr,zephyriot/zephyr,bboozzoo/zephyr,punitvara/zephyr,holtmann/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,zephyriot/zephyr,finikorg/zephyr,punitvara/zephyr,fbsder/zephyr,finikorg/zephyr,runchip/zephyr-cc3200,explora26/zephyr,mbolivar/zephyr,explora26/zephyr