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Use a new environment variable,
ZEPHYR_TOOLCHAIN_SUPPORTS_THREAD_LOCAL_STORAGE, to set the value for
TOOLCHAIN_SUPPORTS_THREAD_LOCAL_STORAGE instead of setting it to 'n' for
all non-Zephyr toolchains. In particular, the Debian arm-none-eabi
toolchain has TLS support and with this option, can be used to build
Zephyr with thread local variables.
Signed-off-by: Keith Packard <keithp@keithp.com>