base-files: functions.sh: fix config_get() on invalid identifiers

base-files: functions.sh: fix config_get() on invalid identifiers

When passing a section or option value to config_get() which contains
characters that happen to be valid variable interpolation expressions,
the function returns a nonsensical expression result instead of the
expected empty string.

When the passed section or option name contains other characters which
are not valid within a shell variable name, a substitution error is
occuring instead.

The issue can be easily reproduced by one of the following examples:

    root@OpenWrt:~# . /lib/functions.sh
    root@OpenWrt:~# config load system
    root@OpenWrt:~# config_get variable invalid-section option
    root@OpenWrt:~# echo "$variable"
    section_option:-

    root@OpenWrt:~# . /lib/functions.sh
    root@OpenWrt:~# config load system
    root@OpenWrt:~# config_get variable section invalid-option
    root@OpenWrt:~# echo "$variable"
    option:-

    root@OpenWrt:~# . /lib/functions.sh
    root@OpenWrt:~# config load system
    root@OpenWrt:~# config_get variable section invalid@option
    -ash: eval: syntax error: bad substitution

Fix this issue by only performing interpolations when the given section
and option arguments are free of illegal characters.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>