package/swupdate: bump to version 2019.04

package/swupdate: bump to version 2019.04

New features in this release:
 - Improved documentation
 - delta updates based on rdiff library
 - support for libubootenv
 - dry-run option
 - CA certificates for signed images
 - Fix security leak in parser

This commit also:
 - introduce BR2_PACKAGE_LIBRSYNC for 'rdiff' Handler. No HAVE_* is
   needed, it just declares the functions locally and links with
   -lrsync.
 - introduce BR2_PACKAGE_LIBUBOOTENV
 - drop upstreamed patch
 - backport upstream patches (important fix)
 - regenerate the default swupdate.config. Now CONFIG_GUNZIP is always
   enabled because gunzip is provided by the default busybox config.

Signed-off-by: Pierre-Jean Texier <pjtexier@koncepto.io>
[Arnout: don't mention ZLIB in help text for libubootenv, since it is
 select'ed by libubootenv.]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>