Switch raspi3 builds to the linux-raspi-hwe-18.04 kernel

As part of the backport of the 5.4 kernel for Raspberry Pi, the kernel
flavour name is changing to match that in Focal. This is to provide a
consistent name for the 5.4 kernel in both Bionic and Focal.
This effectively rolls the kernel from 5.3 to 5.4 for raspi classic
images.

live-build/auto/config

@@ -906,7 +906,7 @@ case $ARCH in
 				;;
 			raspi3)
 				COMPONENTS='main restricted universe multiverse'
-				KERNEL_FLAVOURS=raspi2-hwe-18.04
+				KERNEL_FLAVOURS=raspi-hwe-18.04
 				add_package install linux-firmware-raspi2 u-boot-rpi flash-kernel u-boot-tools wpasupplicant
 				BINARY_REMOVE_LINUX=false
 				;;