Add `CROSS_TOOLCHAIN_PREFIX`, Better CMake Support

[Alexhuszagh]

Recently, we've updated our images in the main repository to make it easier to use environment variables to access the toolchain, such as exporting CROSS_TOOLCHAIN_PREFIX and always specifying the ar:

• Explicitly set ar for all targets. cross#1138
• Export and use CROSS_TOOLCHAIN_PREFIX. cross#1126

Once CMake support is supported, we should also ensure these images also use our CMake toolchain files or variants of it:

• Fix CMake Support and Enhance Newlib Targets cross#1112