meta-agl-bsp: NXP i.MX6 and i.MX8 fixes

These changes should allow the etnaviv GPU driver using NXP i.MX6
and i.MX8 machines (e.g. cubox-i, imx8mq-evk) to build.  Testing on
hardware is required for the i.MX6, and etnaviv does not seem to be
working on the i.MX8MQ-EVK as the previous devicetree tweak being
used no longer seems applicable.  Further investigation will be
done in the near future as time permits.

Notable changes:
- The i.MX6 configurations have been switched to using the linux-fslc
  kernel instead of linux-fslc-lts, as the latter is older (6.1 vs
  6.6).
- The kernel patch against linux-fslc-imx to enable the MHDP driver
  when using etnaviv has been removed since it is no longer required.
- The local workaround to allow enabling the "wayland" PACKAGECONFIG
  when building weston-imx has been replaced with use of the now
  provided upstream variable that allows that.  Note that building
  with this version of Weston has not been tested since it is tied to
  using the Vivante GPU driver.

Bug-AGL: SPEC-5122

Change-Id: I75810dd152197f9435158839fad00327ed7e42e7
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/29878
ci-image-build: Jenkins Job builder account
Tested-by: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>