rootfs.py: Run depmod(wrapper) against each compiled kernel

rootfs.py: Run depmod(wrapper) against each compiled kernel

We run depmod (through depmodwrapper) at the end of the rootfs
generation process. This part of the process assumes in its current
implementation that the kernel package name is always 'kernel' and that
there is only one set of kernel modules for which we need to generate
the modules.dep and map files.

The kernel package name can be configured via a variable
(KERNEL_PACKAGE_NAME) and becomes a namespace that enables the build
system to deal with multiple compiled kernel recipes. This patch checks
for all the depmod pkgdata and runs depmod for each of the detected
kernel versions/kernel package name.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>