kernel: Add KERNEL_IMAGETYPES to build multi types kernel at one time

Add KERNEL_IMAGETYPES to support building packaging and installing
multi types of kernel images, such as zImage uImage, at one time.

KERNEL_IMAGETYPE and KERNEL_ALT_IMAGETYPE work as before.

(From OE-Core rev: 849b67b2e4820564b5e5c9bd4bb293c44351c5f3)

Change-Id: I23e413e3c950f6daed923c69b882993a214dd49c
Signed-off-by: He Zhe <zhe.he@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Rick Altherr <raltherr@google.com>
diff --git a/import-layers/yocto-poky/meta/conf/documentation.conf b/import-layers/yocto-poky/meta/conf/documentation.conf
index 4234d75..dcc324c 100644
--- a/import-layers/yocto-poky/meta/conf/documentation.conf
+++ b/import-layers/yocto-poky/meta/conf/documentation.conf
@@ -245,6 +245,7 @@
 KERNEL_EXTRA_ARGS[doc] = "Specifies additional make command-line arguments the OpenEmbedded build system passes on when compiling the kernel."
 KERNEL_FEATURES[doc] = "Includes additional metadata from the Yocto Project kernel Git repository. The metadata you add through this variable includes config fragments and features descriptions."
 KERNEL_IMAGETYPE[doc] = "The type of kernel to build for a device, usually set by the machine configuration files and defaults to 'zImage'."
+KERNEL_IMAGETYPES[doc] = "The list of types of kernel to build for a device, usually set by the machine configuration files and defaults to KERNEL_IMAGETYPE."
 KERNEL_MODULE_AUTOLOAD[doc] = "Lists kernel modules that need to be auto-loaded during boot"
 KERNEL_MODULE_PROBECONF[doc] = "Lists kernel modules for which the build system expects to find module_conf_* values that specify configuration for each of the modules"
 KERNEL_PATH[doc] = "The location of the kernel sources. This variable is set to the value of the STAGING_KERNEL_DIR within the module class (module.bbclass)."