[build] Construct all-drivers list based on driver class

[build] Construct all-drivers list based on driver class

The USB bus drivers (ehci.c and xhci.c) have PCI device ID tables and
hence PCI_ROM() lines, but should probably not be included in the
all-drivers build on this basis, since they do nothing useful unless a
USB network driver is also present.

Fix by constructing the all-drivers list based on the driver class
(i.e. the portion of the source path immediately after "drivers/").

Signed-off-by: Michael Brown <mcb30@ipxe.org>