How do I include the linux kernel headers as part of the SDK package in Yocto?

I'm using Yocto 1.8 (fido) in an embedded project and want to do out-of-tree kernel module development. Currently, I can build my kernel modules (apart from bitbake) by pointing my $KERNEL_PATH to the poky/build/tmp/work-shared//kernel-source/ directory when I run make. I don't want to do this in the long term, however, since others need to easily build the modules without installing and building a full image from bitbake.

I can generate an SDK using bitbake myimage -c populate_sdk. However, this doesn't include the kernel headers (all I ever see is sysroots//usr/include/linux). How do I cause the kernel headers to be included in the SDK package? Also, I don't want the kernel headers to show up as part of my target image.

[Edit] My image recipe is as follows:

EXTRA_IMAGE_FEATURES_append = " eclipse-debug debug-tweaks"
TOOLCHAIN_HOST_TASK_append = " nativesdk-cmake"
IMAGE_INSTALL = "packagegroup-core-boot  ${CORE_IMAGE_EXTRA_INSTALL} util-linux kernel-modules netbase busybox base-passwd base-files sysvinit initscripts bash gdbserver strace sysfsutils dtc gawk ethtool grep sed wget  iptables oprofile net-tools dropbear rsync stress-ng rt-tests i2c-tools"
inherit core-image

The kernel I'm using is linux-altera-ltsi-rt in the meta-altera layer.