linuxboot: Pass $CPUS to edk2/OvmfPkg/build.sh unerroneously

[synackd]

On a clean checkout of HEADS when attempting to build the qemu-linuxboot board, configuration of Linuxboot fails:

Pre-building edk2 OVMF
Initializing workspace
/home/docker/workspace/heads/build/linuxboot-git/edk2/BaseTools
Loading previous configuration from /home/docker/workspace/heads/build/linuxboot-git/edk2/Conf/BuildEnv.sh
WORKSPACE: /home/docker/workspace/heads/build/linuxboot-git/edk2
EDK_TOOLS_PATH: /home/docker/workspace/heads/build/linuxboot-git/edk2/BaseTools
CONF_PATH: /home/docker/workspace/heads/build/linuxboot-git/edk2/Conf
using prebuilt tools
Running edk2 build for OvmfPkgX64
Usage: build.exe [options] [all|fds|genc|genmake|clean|cleanall|cleanlib|modules|libraries|run]

build.exe: error: option -n: invalid integer value: 'PUS'

Changing $$CPUS to $(CPUS) in modules/linuxboot fixes this.

[synackd]

This PR addresses this discussion.