prototypes: xci: xci-deploy: Set default value for devel parameters

Commit 68412ffc734e ("prototypes: xci: xci-deploy.sh: Append trailing
slash for rsync vars") modified xci-deploy.sh script to append a
trailing slash to the devel variables but it broke the script when
one of these variables was not set (since we run with 'set -eu') with
the following error:

./xci-deploy.sh: line 45: !local_user_var: unbound variable

Fixes: I0103b754585931fa1dcd3966c52d7e4a8f2f63f6
Change-Id: Ic2465ff07348b61e74e191516425cd10d80c1fce
Signed-off-by: Markos Chandras <mchandras@suse.de>