open-plc-utils: cleanup package meta-data

open-plc-utils: cleanup package meta-data

Align package variables to other packages which also use a git checkout
as tarball:
 - PKG_VERSION should not be set to a date, just rely on package default
   handling which apply automatically when using PKG_SOURCE_VERSION
 - introduce a plain PKG_RELEASE instead to allow keeping track of modified packaging
 - PKG_SOURCE_DIR is set by package defaults
 - same for PKG_SOURCE
 - this results in a different hash of the tarball created, so adjust
   PKG_MIRROR_HASH

Signed-off-by: Michael Heimpold <redacted>