u-boot.inc: install u-boot-initial-env as ${PN}-initial-env in $D and $DEPLOYDIR

The common u-boot.inc can be used by multiple recipes in the same build for
different cores and/or multiple stages of the bootloader. Naming initial-env
with ${PN} prefix avoids clashes in deploy and rootfs between those recipes.

This fixes 69b3b093 that unconditionally
builds, installs and deploys u-boot-initial-env in the common u-boot.inc.

Signed-off-by: Denys Dmytriyenko <denys@ti.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>