Rename output directory, patch flash script

- Rename output directory from imx8m_phanbell=>imx8mm_columbia
- Update flash script with new output path, disable reboot-bootloader
until it's implemented in u-boot
- Remove imx-board-wlan (QCA chip not present in columbia)

Change-Id: Ib2a83a46f7d8f8fb1ea4e4680a29b124c0ca3d0c