kbuild: Make the setlocalversion script POSIX-compliant

The 'source' builtin is a bash alias to the '.' (dot) builtin. While the
former is supported only by bash, the latter is specified in POSIX and
works fine with all POSIX-compliant shells I am aware of.

The '$_' special parameter is specific to bash. It is partially
supported in dash too but it always evaluates to the current script path
(which causes the script to enter a loop recursively re-executing
itself). This is why I have replaced the two occurences of '$_' with the
explicit parameter.

The 'local' builtin is another example of bash-specific code. Although
it is supported by all POSIX-compliant shells I am aware of, it is not
part of POSIX specification and thus the code should not rely on it
assigning a specific value to the local variable. Moreover, the 'posh'
shell has a limited version of 'local' builtin not supporting direct
variable assignments. Thus, I have broken one of the 'local'
declarations down into a (non-POSIX) 'local' declaration and a plain
(POSIX-compliant) variable assignment.

Signed-off-by: Michał Górny <gentoo@mgorny.alt.pl>
Signed-off-by: Michal Marek <mmarek@suse.cz>