Ok, I have found the source.

It is part of the modules-init-tools project.

The problem with the depmod utility is as follows:

There does not appear to be an option to prevent depmod from appending
/lib/modules/2.x.x to the output path when using the -b option.

For example:

depmod -ae -F System.map -b /u1/build/kmodules -r 2.6.0

gives error:

depmod: Can't open /u1/build/kmodules/lib/modules/2.6.0/modules.dep
for writing.

The directory /u1/build/kmodules exists with read,write,execute
permission.

The de

lib/modules/2.6.0, which does not exist.

This is causing the kernel building process to fail.

--

From man page:
-b basedir --basedir basedir
If  your  modules are not currently in the (normal) directory
/lib/modules/version, but in a staging area, you can  specify
a  basedir  which  is  prepended to the directory name.  This
basedir is stripped from the resulting modules.dep  file,  so
it is ready to be moved into the normal location.