Add MXS bootlets package

Add MXS bootlets package

MXS platforms (imx23 and imx28) are relying on bootlets as their first
stage bootloaders, that can then either start a regular second stage
bootloader or directly a Linux kernel. However, the Makefile allows
only to build u-boot and linux images at the same time, which is
not very convenient as we will more likely use only one of them,
so we need to duplicate a bit what is already done so that we are able
to choose what we want to generate.

thomas.petazzoni@free-electrons.com:

 * Remove incorrect dependency on BR2_PACKAGE_ELFTOSB

 * Each board configuration option is for one board, not multiple
   boards, so use singular.

 * The i.MX28 support is for i.MX28 EVK only, reflect that in the
   option prompt and the option name.

 * Use 'generic-package' instead of GENTARGETS

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>