modules/linuxboot: Fix qemu build after c745640

> 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'

Signed-off-by: Nathan Rennie-Waldock <nathan.renniewaldock@gmail.com>

Author: NHellFire

modules/linuxboot

@@ -21,7 +21,7 @@ linuxboot_configure := \
 	if [ "$(linuxboot_board)" = "qemu" ]; then \
 		echo >&2 "Pre-building edk2 OVMF" ; \
 		( cd $(build)/$(linuxboot_base_dir)/edk2/OvmfPkg ; \
-		./build.sh -n $$CPUS \
+		./build.sh -n $(CPUS) \
 		) || exit 1 ; \
 	fi ; \
 	touch .config ; \