Add 'bc' to files/apts/general

After commit def4c141, "bc" command is used in stack.sh,
but 'bc' command is not available in very minimal Ubuntu
installation (without any tasks installed).
We need to add 'bc' to required package list.

Closes-Bug: #1272914
Change-Id: I5797707e8eaa9dd2a21d1a1fc3af028d1951a2ee
Merge "Add 'bc' to files/apts/general"