package_ipk.bbclass: make DESCRIPTION support newline

package_ipk.bbclass: make DESCRIPTION support newline

The recipe's DESCRIPTION is wrapped automatically by textwrap, make it
support newline ("\n") to let the user can wrap it manually, e.g.:

DESCRIPTION = "Foo1\nFoo2"

In the past, it would be:
Foo1\nFoo2

Now:
Foo1
Foo2

[YOCTO #4348]

(From OE-Core rev: dff04de2de8bb159fd6912e29794eadd75d5d92a)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>