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>

target/linux/generic/config-5.10

@@ -2873,6 +2873,7 @@ CONFIG_KERNEL_XZ=y
 CONFIG_KERNFS=y
 # CONFIG_KEXEC is not set
 # CONFIG_KEXEC_FILE is not set
+# CONFIG_KEXEC_SIG is not set
 # CONFIG_KEYBOARD_ADC is not set
 # CONFIG_KEYBOARD_ADP5588 is not set
 # CONFIG_KEYBOARD_ADP5589 is not set

target/linux/generic/config-5.4

@@ -2613,6 +2613,7 @@ CONFIG_KERNEL_XZ=y
 CONFIG_KERNFS=y
 # CONFIG_KEXEC is not set
 # CONFIG_KEXEC_FILE is not set
+# CONFIG_KEXEC_SIG is not set
 # CONFIG_KEYBOARD_ADC is not set
 # CONFIG_KEYBOARD_ADP5588 is not set
 # CONFIG_KEYBOARD_ADP5589 is not set

target/linux/rockchip/armv8/config-5.10

@@ -298,7 +298,6 @@ CONFIG_JUMP_LABEL=y
 CONFIG_KALLSYMS=y
 CONFIG_KEXEC_CORE=y
 CONFIG_KEXEC_FILE=y
-# CONFIG_KEXEC_SIG is not set
 CONFIG_KSM=y
 # CONFIG_LEDS_BRIGHTNESS_HW_CHANGED is not set
 CONFIG_LEDS_GPIO=y