classes/kernel: No symlink in postinst without KERNEL_IMAGETYPE_SYMLINK

The commit “Use a copy of image for kernel*.rpm if fs doesn't support
symlinks” [1] added postinst and postrm scripts to the kernel package which
create a symlink after package installation. This should not happen if
`KERNEL_IMAGETYPE_SYMLINK` is not `1`.

Background: The u-boot implementation of jffs2 does not support symlinks.
Using a hardlink or removing `${KERNEL_VERSION}` from the file name fails,
because the current postinst script replaces the file with the symlink.

[1] 8b6b95106a5d4f1f6d34209ec5c475c900270ecd

Cc: Bruce Ashfield <bruce.ashfield@gmail.com>
Cc: Richard Purdie <richard.purdie@linuxfoundation.org>
Cc: Yanfei Xu <yanfei.xu@windriver.com>
Signed-off-by: Jörg Sommer <joerg.sommer@navimatix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>