Add s390 support for linuxkit kernel

Update building process to add s390 support.

The patch serial-forbid-8250-on-s390.patch has been added to disable
8250 serial for s390.

The patch is available upstream https://patchwork.kernel.org/patch/10106437/
but it is not backported.

Signed-off-by: Alice Frosi <alice@linux.vnet.ibm.com>

kernel/Dockerfile

@@ -91,6 +91,9 @@ RUN case $(uname -m) in \
     aarch64) \
         KERNEL_DEF_CONF=/linux/arch/arm64/configs/defconfig; \
         ;; \
+    s390x) \
+        KERNEL_DEF_CONF=/linux/arch/s390/defconfig; \
+        ;; \
     esac  && \
     cp /config-${KERNEL_SERIES}-$(uname -m) ${KERNEL_DEF_CONF}; \
     if [ -n "${EXTRA}" ] && [ -f "/config-${KERNEL_SERIES}-$(uname -m)${EXTRA}" ]; then \
@@ -116,6 +119,9 @@ RUN make -j "$(getconf _NPROCESSORS_ONLN)" KCFLAGS="-fno-pie" && \
     aarch64) \
         cp arch/arm64/boot/Image.gz /out/kernel; \
         ;; \
+    s390x) \
+        cp arch/s390/boot/bzImage /out/kernel; \
+        ;; \
     esac && \
     cp System.map /out && \
     ([ -n "${DEBUG}" ] && cp vmlinux /out || true)