package/swupdate: add libubootenv as optional dependency

If the swupdate configuration contains CONFIG_UBOOT=y it uses
libubootenv to access the U-Boot environment.

We don't have Buildroot config options for all the different optional
dependencies of swupdate, instead we rely on the user to select the
appropriate packages and simply add the dependency in the .mk file. Do
this for libubootenv as well. swupdate doesn't have anything like
HAVE_LIBUBOOTENV, it just assumes libubootenv is available.

Fixes:
bootloader/uboot.c:23:10: fatal error: libuboot.h: No such file or directory
   23 | #include <libuboot.h>

Note that libubootenv is normally built before swupdate (alphabetical
ordering), so the error only occrus with BR2_PER_PACKAGE_DIRECTORIES or
when building swupdate directly.

Note that the autobuilders don't have this error, because they only
build swupdate with a default configuration that doesn't have U-Boot
support.

Signed-off-by: Michael Nosthoff <buildroot@heine.tech>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>