Add 'bc' in the mandatory dependencies

Since a few kernel releases, having 'bc' installed is mandatory to
build the kernel. See commit
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=70730bca1331fc50c3caacaea00439de1325bd6e


of the kernel.

Since this tool is generally available by default in distributions,
and we're unlikely to see version-specific problems with it, we
just check for it to be installed in
support/dependencies/dependencies.sh.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>