u-boot.mk: do not force parallel build

Some patched u-boots may have problems with parallel build.
Do not enforce parallel build here so one can set PKG_BUILD_PARALLEL:=0
in the specific u-boot Makefile also before including the u-boot.mk.

Signed-off-by: Martin Schiller <ms@dev.tdt.de>

include/u-boot.mk

@@ -16,7 +16,7 @@ PKG_FLAGS:=nonshared
 PKG_LICENSE:=GPL-2.0 GPL-2.0+
 PKG_LICENSE_FILES:=Licenses/README
 
-PKG_BUILD_PARALLEL:=1
+PKG_BUILD_PARALLEL ?= 1
 
 export GCC_HONOUR_COPTS=s