apalis/colibri_imx6/-imx6ull/_imx7: update env memory layout

Update the distro config env memory layout for the Apalis iMX6,
Colibri iMX6, Colibri iMX6ULL and Colibri iMX7:

- loadaddr=0x84200000 (resp. 0x14200000 on them i.MX 6) allows for 64MB
  area for uncompressing (ie FIT images)
- fdt_addr_r = loadaddr + 64MB : allows for 64MB kernel
- scriptaddr = fdt_addr_r + 512KB : allows for 512KB fdt
- pxefile_addr_r = scriptaddr + 512KB : allows for 512KB script
- ramdisk_addr_r = pxefile_addr_r + 512KB : allows for 1MB extlinux.conf

Memory layout analogous to 64-bit one from commit fd5c7173
("imx8m{m,n}_venice: update env memory layout") but left pxefile_addr_r
updated according to doc/develop/distro.rst.

This fixes a potential issue caused by the compressed kernel being
relocated on top of the ramdisk causing its corruption.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>