kernel: include CONFIG_KEXEC_SIG in configs

kernel: include CONFIG_KEXEC_SIG in configs

Seeing failure to build because of missing symbols related to provisioning
CONFIG_KEXEC and signed images.  Without this, if you set
CONFIG_KERNEL_KEXEC=y and try to build, target/linux will hang at:

scripts/kconfig/conf  --syncconfig Kconfig
...
kexec system call (KEXEC) [Y/n/?] y
kexec file based system call (KEXEC_FILE) [Y/n/?] y
Verify kernel signature during kexec_file_load() syscall (KEXEC_SIG) [N/y/?] (NEW)

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>