roms/edk2-funcs.sh: add the qemu_edk2_get_thread_count() function
The edk2 "build" utility natively supports building modules (that is, INF
files) in parallel. The feature is not useful when building a single
module (with the "-m" option), but it is useful for platform firmware
builds (which include many modules). Add a function that determines the
"-n" option argument for "build", from the MAKEFLAGS variable (i.e. based
on the presence of a make job server).