package.bbclass: add getter of additional metadata

package.bbclass: add getter of additional metadata

Two variables are searched for value of additional package metadata:

  * PACKAGE_ADD_METADATA_<PKGTYPE>
  * PACKAGE_ADD_METADATA

First found variable with defined value wins.

<PKGTYPE> is a parameter of getter and expected to be a distinct name
of specific package type. For example: 'DEB' or 'RPM'.

Variable can contain multiple [one-line] metadata fields, separated by
literal sequence '\n'. Separator can be redefined through variable flag
'separator'. Getter returns found value with separator replaced with
newline character.

As side-effect, searched variables acquired flags 'type' (equals to
'list') and 'separator'.

(From OE-Core rev: 98ea2fc35a3ef609a944929e21e0f9be2889036d)

Signed-off-by: Leonid Borisenko <ive.found@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>