image_types_fsl.bbclass: add imx-bootlets support for i.MXS
i.MXS SoC family has support to boot with imx-bootlets (the default
when using LTIB) and the only current supported way of booting i.MX23
machines.
The imx-bootlets has some quite specific requirements regaring the
partition layout and includes the kernel binary appended to the boot
stream so we drop the third partition reducing the wasted space of SD
card.
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
