CONFIG_UML_NET_PCAP is not available for thedefaultarchitecture x86.
Result is shown for architecture um

pcap transport (obsolete)

modulename: pcap.ko

configname: CONFIG_UML_NET_PCAP

Linux Kernel Configuration
└─>UML Network Devices
└─>pcap transport (obsolete)
In linux kernel since version 2.6.12  
The pcap transport makes a pcap packet stream on the host look
like an ethernet device inside UML. This is useful for making
UML act as a network monitor for the host. You must have libcap
installed in order to build the pcap transport into UML.

For more information, see
<http://user-mode-linux.sourceforge.net/old/networking.html> That site
has examples of the UML command line to use to enable this option.

NOTE: THIS TRANSPORT IS DEPRECATED AND WILL BE REMOVED SOON!!! Please
migrate to UML_NET_VECTOR.

If unsure, say N.

source code: