samples: basic: threads: Rework to not define generated macros

Rather than redefining a macro which might be generated we should create
a new define name that should get used in the code.  Replace redefining
DT_ALIAS_LED0_GPIOS_FLAGS with LED0_FLAGS and DT_ALIAS_LED1_GPIOS_FLAGS
with LED1_FLAGS.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>