pkg-utils: kconfig helpers: use single iso double quoting

The echo statements in the kconfig helpers are currently using double
quotes. For KCONFIG_SET_OPT this is problematic when the value argument
itself contains a double quote (a string value). In this case, the statement
    echo "$(1)=$(2)" >> $(3)
would become:
    echo "FOO="string value"" >> /some/path/.config
resulting in the string
    FOO=string value
in the config file, rather than the properly quoted
    FOO="string value"

The linux package worked around this by escaping the quote characters, but
a prettier solution is to use single quoting in the helpers (or
alternatively use no quoting at all).
A side effect of this change is that a $variable in the key or value would
no longer be interpreted by the shell, removing any unexpected behavior.

This change is only really necessary for KCONFIG_SET_OPT, but for symmetry
reasons the other helpers are updated too.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>