buildroot

uboot-tools: install libubootenv to staging

libubootenv is required by swupdate.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
[Thomas: do the installation directly in <pkg>_INSTALL_STAGING_CMDS.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>