jool: fix PKG_BUILD_DIR to avoid kernel ABI mismatch

As jool builds a kernel module, a PKG_BUILD_DIR under KERNEL_BUILD_DIR must
be used to avoid reusing build artifacts when switching to a different
target of the same architecture. Otherwise, kernel ABI mismatches may
result, leading to an unusuable module, or build failures like the
following:

    Package kmod-jool is missing dependencies for the following libraries:
    crypto_hash.ko

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

net/jool/Makefile

@@ -8,7 +8,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=jool
 PKG_VERSION:=2017.03.09
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 
 PKG_LICENSE:=GPL-3.0
 PKG_LICENSE_FILES:=COPYING
@@ -20,7 +20,7 @@ PKG_SOURCE_SUBDIR:=$(PKG_NAME)-$(PKG_SOURCE_VERSION)
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_SOURCE_VERSION).tar.xz
 PKG_MIRROR_HASH:=a2c41119be251d4d962234fc78d7122568a5e62484969cc4e80229a0c7422fd4
 
-PKG_BUILD_DIR:=$(BUILD_DIR)/$(PKG_SOURCE_SUBDIR)
+PKG_BUILD_DIR=$(KERNEL_BUILD_DIR)/$(PKG_SOURCE_SUBDIR)
 PKG_BUILD_PARALLEL:=1
 PKG_BUILD_DEPENDS:=USE_UCLIBC:argp-standalone USE_MUSL:argp-standalone