tools: moveconfig: rename update_defconfig() to update_dotconfig()

Commit 96464bad ("moveconfig: Always run savedefconfig on the
moved config") changed how defconfig files were updated.

Since then, the function update_defconfig() does not modify defconfig
files at all (instead, they are updated by "make savedefconfig"), so
update_dotconfig() is a better fit for this function.  Also, update
the comment block to match the actual behavior.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>