Replace '-' in kernel version with '_'

Removing the '-' results in hard to read filenames such as:
kernel-2.6.35.2000042g76e4caf-28.x86_64.rpm

kernel-2.6.35.2_000042_g76e4caf-28.x86_64.rpm is easier to
read.

Signed-off-by: Arun Sharma <asharma@fb.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>