This will pull in and install 5.4 series kernels for Focal installs,
thanks to the split metapackage logic introduced in #5691

KERNEL_VERSION is defined for each function as required.

CONFIG_IRDA is simply not present in the kernel configuration for 5.4 series kernels, in previous kernel configurations it was commented out.

Discovered by @emkll in [0]. The forked package build logic was
including an empty dir based on the distro string. Update the path
munging logic to use the proper var to avoid.

[0] #5772 (comment)