uclibc: replace custom kconfig helpers with those provided by pkg-utils

This patch removes the custom kconfig helpers UCLIBC_OPT_SET and
UCLIBC_OPT_UNSET with the common KCONFIG_SET_OPT, KCONFIG_ENABLE_OPT and
KCONFIG_DISABLE_OPT.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>