meta-agl-bsp: Update i.MX8MQ EVK support for kirkstone

meta-agl-bsp: Update i.MX8MQ EVK support for kirkstone

Changes:
- Rename machine template and related files to use imx8mq-evk instead
  of imx8mqevk to match meta-freescale change.
- Switch to purely using the default "mainline" BSP configuration in
  meta-freescale for imx8mq-evk as opposed to the tweaking of the
  "nxp" configuration previously used.  The mainline configuration now
  builds a working image, and this lowers maintenance on our part.
- Remove imx8mqevk-viv configuration to build with Vivante GPU driver.
  NXP have not yet released Weston 10 support, and at this point the
  etnaviv driver is stable.  If a demonstration of support of Vivante
  is requested in the future (e.g. for other i.MX8 variants that still
  require it), this can be revisited.

Bug-AGL: SPEC-3819

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I7e279268463f68978ecd2990927aab6d64e3cbd2