buildroot

sam-ba: use HOST_SAM_BA_DL_DIR instead of SAM_BA_DL_DIR

The infrastructure only provides HOST_SAM_BA_DL_DIR, because this
package is host only. Ideally the infra should provide SAM_BA_DL_DIR,
but it doesn't currently, and that requires more significant changes.

Fixes:

  http://autobuild.buildroot.net/results/a7c76138a1cf068868904acb6678403e3b25fcb3/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>