compat: compute approx space used dynamically for bin/get-compat-kernels

compat: compute approx space used dynamically for bin/get-compat-kernels

This assumes about ~ 101 MiB of space is consumed by each kernel
when we uncompress the kernel headers under /usr/src/ and /lib/modules/.
It also assumes each kernel requires two deb files to be downloaded
locally, each kernel then consuming ~ 13 MiB.

When run this currently displays:

mcgrof@tux ~/compat (git::master)$ ./bin/get-compat-kernels
This will download 21 kernel headers to allow you to
cross compile any module over these kernels with ckmake.
The download payload is about ~ 273 MiB, once uncompressed
it will use sudo to stash kernel header files under /usr/src/ and /lib/modules/
and consume about ~ 2121 MiB of space.

The kernel headers used are from Vanilla kernels
from the Ubuntu mainline / vanilla kernel PPA:

http://kernel.ubuntu.com/~kernel-ppa/mainline/

Do you still want to continue (y/N)?

Signed-off-by: Luis R. Rodriguez <mcgrof@frijolero.org>