meta-phosphor: set DISTRO_CODENAME

Back in 2018 we stopped including poky.conf by default and treated
phosphor as a separate distro.  One minor side-effect of this is that
the DISTRO_CODENAME is no longer defined, which is used in some misc.
locations such as os-release.  Mirror the current Poky codename with
the intention that we will keep this in sync as upstream moves along.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Iabbdd3f6046f2c7b8ffc81536ab4b47f8471ee20