torture: Permit double-quoted-string Kconfig options

Currently, the presence of any quoted-string Kconfig option in the
scenario files or the CFcommon file (aside from the special-cased
CONFIG_INITRAMFS_SOURCE option) will result in an "improperly set"
diagnostic.  This commit updates configcheck.sh to strip double quotes
in order to permit string-valued Kconfig options to be handled correctly.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>