u-boot: Allow for customizing installed/deployed file names

u-boot: Allow for customizing installed/deployed file names

When assembling all of the various filenames that are installed/deployed
from u-boot, we have been including the PV and PR in the filenames.  This
change introduces a single variable to replace these two in the
filenames.

This change should not be disruptive since the default value for the new
UBOOT_VERSION variable is "${PV}-${PR}".

In one case (UBOOT_EXTLINUX_SYMLINK [1]), PR was used without PV, this
patch assumes this was a mistake and corrects it as PR would not be of
much use alone.

[1] https://git.openembedded.org/openembedded-core/commit/?h=master-next&id=33df3a65f3e8e136811da715d0cc247ce66ae0ea

Signed-off-by: Ryan Eatmon <reatmon@ti.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>