deb: apply postinstall on sdk

If not postinstall applied, some nativesdk command could not be found
in sdk due to update-alternatives in postinst not be executed, such as chroot:

$ which chroot
/sbin/chroot
$ which chroot.coreutils
path-to-sdk/sysroots/x86_64-wrlinuxsdk-linux/usr/bin/chroot.coreutils

After applying the fix
$ which chroot
path-to-sdk/sysroots/x86_64-wrlinuxsdk-linux/usr/bin/chroot
$ which chroot.coreutils
path-to-sdk/sysroots/x86_64-wrlinuxsdk-linux/usr/bin/chroot.coreutils

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 2a9bf195)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>