package/pkg-utils: refine KCONFIG_MUNGE_DOT_CONFIG

Given a .config with:

    FOO="1234"
    BAR="$(FOO)"

and then:

    $(call KCONFIG_SET_OPT,FOO,azerty)

would yield a .config with just:

    FOO="azerty"

because \<FOO\> would match the assignment to BAR.

Reported-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: TIAN Yuanhao <tianyuanhao3@163.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>