poplar: misc updates needed for Linux 5.5

1. Drop workarounds for GCC 8.x
2. Provide eMMC as env variable, which is used in extlinux.conf
distroboot script.
3. Use default defconfig for all arm64.
4. Merge additional poplar.conf

Acked-by: Jerome Forissier <jerome@forissier.org>
Signed-off-by: Igor Opaniuk <igor.opaniuk@gmail.com>