extract_storage: change build number naming scheme for feature branch

extract_storage: change build number naming scheme for feature branch

This is only a temporary solution to resolve build number conflicts
on Gentoo systems. Without the change, it's nearly impossible to manage
different ebuilds for both master/develop branches and feature branches.
For the example above, new ebuilds for feature branch (build no. 150)
should be recognized as a newer version than the last develop kernel.
In reality, however, the develop kernel has its build number 395, and
the feature build number 150 will be recognized as its previous version.

To avoid such confusion, change build number naming scheme,
from a simple number to a more precise string according to a format

  $(last_develop_buildno).$(feature_buildno)

, where $(last_develop_buildno) is the most recent build number
in the develop branch, and $(feature_buildno) is the current feature
branch's build number. For example, if the last build number of develop
branch is 395, and that of feature branch is 150, then the output string
will be "395.150".