package/freescale-imx/firmware-imx: refine vpu and sdma copies

package/freescale-imx/firmware-imx: refine vpu and sdma copies

In latest patch the SDMA installation was limited to platforms whose
name was mentioned in the binary but this approach wasn't flexible to
manage cases like imx8m using imx7 sdma binary, so this patch does:
- change VPU/SDMA FW options to string to allow specifying the name
  (suggested by Thomas)
- remove imx27 sdma case as non-existent
- add imx8m family support (using imx7d binary)
- get rid of FIRMWARE_IMX_PLATFORM_LOWER macro

Fixes: fad2df39b9 ("package/freescale-imx/firmware-imx: clarify installation of firmware files")
Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>