kernel.bbclass: Do not overwrite recipe's custom postinst

Allow recipe to set a custom postinst for kernel image.
Use prependVar in order to allow custom postinst to be
ran after default postinst, potentially modifying the
installed file/symlink.

Signed-off-by: Raphael Teller <raphael.teller@se.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>