base-files: introduce a function to get kernel version number

base-files: introduce a function to get kernel version number

A new function "get_linux_version()" to normalize and print the
kernel version as an integer. In some migration scripts, it is
useful for checking the Linux kernel version.

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/19172
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit a0fe3cfb62fbf0b5f599c90e488727f493223429)
Link: https://github.com/openwrt/openwrt/pull/20739
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>