build-atf, build-u_boot: Use nproc to figure out parallelism

.github/workflows/ci.yml

@@ -13,9 +13,6 @@ permissions:
       id-token: write
       contents: read
 
-env:
-  MAKEFLAGS: -j2
-
 jobs:
   build-boot-x86:
     runs-on: ubuntu-latest

scripts/build-atf

@@ -32,4 +32,4 @@ fi
 export CROSS_COMPILE=$(dirname $(which "${TUPLE}-gcc"))/"${TUPLE}-"
 
 # Build ATF
-make ${MAKEFLAGS} PLAT="${PLAT}" DEBUG=1 bl31
+make ${MAKEFLAGS:--j$(nproc)} PLAT="${PLAT}" DEBUG=1 bl31

scripts/build-u_boot

@@ -32,4 +32,4 @@ fi
 export CROSS_COMPILE=$(dirname $(which "${TUPLE}-gcc"))/"${TUPLE}-"
 
 # Build U-Boot
-make ${MAKEFLAGS} "${DEFCONFIG}" && make ${MAKEFLAGS}
+make ${MAKEFLAGS} "${DEFCONFIG}" && make ${MAKEFLAGS:--j$(nproc)}