Update i.MX machine configurations

Update i.MX machine configurations

Changes:
- Add new i.MX and i.MX6 common inc files and shift duplicated
  configuration to them.
- Switch to using the newer IMX_BSP_DEFAULT variable mechanism for
  selecting mainline vs NXP bits, instead of adding use-mainline-bsp
  to MACHINEOVERRIDES.
- Remove the accidental addition of agl-medium-arm-compiler to
  DISTRO_FEATURES for the imx8mqevk configurations.
- Remove some other stale variable definitions.
- Update BSP layer path in i.MX6 template 50_setup.sh files, these
  were missed on a previous update.

Bug-AGL: SPEC-3933

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I0145e3c8b73b4d53706fda4237ab10f181fd16f7
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/26377
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>