Create option to modify the toolchain PREFIX for cmake

[KeilChris]

The Zephyr SDK contains a GCC toolchain that differs from the GNU Arm Embedded toolchain in the way that the executables are not called arm-none-eabi- but arm-zephyr-eabi-. Currently, the GCC.10.3.1.cmake file has this hardcoded:

if(DEFINED TOOLCHAIN_ROOT)
  set(PREFIX arm-none-eabi-)

Thus, a compilation with the Zephyr SDK will fail as the wrong exeutable is called:

Loading Zephyr default modules (Zephyr base (cached)).
CMake Error at /Users/user/zephyrproject/zephyr/cmake/compiler/gcc/generic.cmake:9 (message):
  Zephyr was unable to find the toolchain.  Is the environment misconfigured?

  User-configuration:

  ZEPHYR_TOOLCHAIN_VARIANT: gnuarmemb

  Internal variables:

  CROSS_COMPILE:
  /Users/user/zephyr-sdk-0.17.4/arm-zephyr-eabi//bin/arm-none-eabi-

  TOOLCHAIN_HOME: /Users/user/zephyr-sdk-0.17.4/arm-zephyr-eabi/

A possible solution could be that we honor an environment variable (for example TOOLCHAIN_PREFIX) that can be set by the user if they encounter such a problem.