Avoid overriding DISTRO_NAME in tn-base.inc

[lgrosz]

Assumedly, tn-base.inc is supposed to be included to support any Technexion board/SOM (correct me if I'm wrong). Setting DISTRO_NAME:tn and performing OVERRIDES_append = ":tn" in this file causes distributions setting DISTRO_NAME expectedly to have the wrong DISTRO_NAME after parsing.

Since tn-base.inc is machine configuration, not distro configuration, can this line be removed from this layer?

[lgrosz]

For now a work around is to perform an unset after including tn-base.inc, but this isn't ideal since it pollutes machine configuration with distro variables:

meta-my-bsp/conf/machine/my-tn-based-machine.conf

# includes not located in expected place
# require conf/machine/include/tn-base.inc
require conf/machine/tn-base.inc

# distro name shouldn't be set by machine configuration
unset DISTRO_NAME:tn