Some packages such as device drivers and HAL packages are hardware-specific, and generally it makes no sense to add such packages to a configuration unless the corresponding hardware is present on your target system. Typically hardware package selection happens automatically when you select your target. The hardware property can be used in the body of a cdl_package command to indicate that the package is hardware-specific.
Note: At the time of writing the hardware property is largely ignored by the configuration tools, but this may change in future. Amongst other possible changes, for hardware-specific packages such as device drivers and HAL packages, the current scheme of generating a configuration header file name based purely on the package name may be abandoned. Instead all hardware packages would send their configuration data to a single header file, pkgconf/hardware.h. This would make it easier for code to obtain details of the current hardware, but obviously there are compatibility issues. For now it is recommended that all hardware packages specify their configuration header file explicitly.