kconfig: do not override symbols already set
kconfig: do not override symbols already set

For randconfig, if a list of required symbols is specified with
KCONFIG_ALLCONFIG, such symbols do not "have a value" as per
sym_has_value(), but have the "valid" flag set.

Signed-off-by: "Yann E. MORIN" <b375a36861a21dce61868a5a9b415811d17b4e97@free.fr>

KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas