firefox: allow limiting concurrent jobs

The PARALLEL_MAKE OE variable has been ignored until now, Firefox
was built with the number of jobs determined by mach (which is usually
the number of CPU cores).

To be able to control this number, pass the PARALLEL_MAKE variable value
as a custom make flag to mach.

Signed-off-by: Gyorgy Sarvari <skandigraun@gmail.com>

Author: OldManYellsAtCloud

meta-firefox/classes/mozilla.bbclass

@@ -36,6 +36,8 @@ export WASM_CXX = "${WASI_SYSROOT}/bin/clang++ -target wasm32-wasip1 "
 
 export BUILD_VERBOSE_LOG = "1"
 
+export MOZ_MAKE_FLAGS = "${PARALLEL_MAKE}"
+
 RUST_HOST ?= "${RUST_BUILD_SYS}"
 RUST_TARGET ?= "${RUST_HOST_SYS}"