doc: board: apalis-imx8: Update and improve documentation

Update and improve the building documentation of Apalis iMX8.
The following changes were made:
- imx-atf repository changed to nxp-imx GitHub.
- imx-atf branch updated to 'lf_v2.6'.
- imx-seco updated to version 5.8.7.
- nxp-imx mfgtools link updated to GitHub releases.
- General writing improvements.

Signed-off-by: Hiago De Franco <hiago.franco@toradex.com>