scripts/extract/globals: treat node alternate names as 'aliases'

Treat new generated dtc element 'alt-name' as usual aliases.
Aim is not to duplicate generation of node elements but provide
pointers using generic alternate names.
For instance, defining following overriding node:
arduino_i2c : i2c1 {};
Will generate following definition:
#define ARDUINO_I2C_LABEL	ST_STM32_I2C_V1_40005400_LABEL

Then 'ARDUINO_I2C_LABEL' could be use a generic binding.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>