ARM: imx6: mx6sabre-common: Search for boot components in /boot too

ARM: imx6: mx6sabre-common: Search for boot components in /boot too

Some root filesystem configurations do not have separate /boot partition,
but rather place kernel, DT, scripts into /boot directory. Search the /boot
directory for these boot components in case they are not found in /, which
is the old behavior.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Ye Li <ye.li@nxp.com>
Cc: uboot-imx <uboot-imx@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>