firmware-imx: move to the freescale-imx directory

This commit moves the firmware-imx package to the newly introduced
freescale-imx/ directory, and uses the new variables provided by
freescale-imx/freescale-imx.mk.

Signed-off-by: Henk Fijnvandraat <h.fijnvandraat@inter.nl.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>