Fix the ftp-stage target for arm embedded builds.

The images were renamed from KERNCONF to BOARDNAME when
specified, which would result in an image name of:

12.0-CURRENT-arm-armv7-GENERIC.img

which would then be renamed to use the BOARDNAME for the
SoC the image is targeted to use. BOARDNAME was specified
for all images as of r336994, which now causes the ftp-stage
target to fail, as the rename is no longer necessary.

Sponsored by: The FreeBSD Foundation