openblas: avoid using make -j when building

Set MAKE_NB_JOBS=-1 so that no -j parameter is passed to make when
building the package, honoring the parent make's setting.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>

libs/openblas/Makefile

@@ -81,6 +81,7 @@ MAKE_FLAGS += \
 	HOSTCC=$(HOSTCC_NOCACHE) \
 	CROSS_SUFFIX=$(TARGET_CROSS) \
 	BINARY=$(OPENBLAS_BINARY) \
+	MAKE_NB_JOBS=-1 \
 	NUM_THREADS=2 \
 	PREFIX=/usr \
 	COMMON_OPT="" \