package/linuxptp: set KBUILD_OUTPUT to STAGING_DIR

incdefs.sh try to define some flags with user_flags() and kernel_flags()
functions. The later is looking at the kernel headers installed on the host
when KBUILD_OUTPUT is not set. If no kernel headers are installed on the host,
the grep fail and HAVE_ONESTEP_SYNC is not set on the command line:
see: grep: /usr/include/linux/net_tstamp.h: No such file or directory
So the missing.h define HWTSTAMP_TX_ONESTEP_SYNC which is also present in the
kernel headers installed in STAGING_DIR (toolchain w/ headers >= 3.2).

Indeed KBUILD_OUTPUT is empty because it's reset in the makefile, so move
KBUILD_OUTPUT in the enviroment while calling "make"/

Also set KBUILD_OUTPUT to STAGING_DIR to find net_tstamp.h.

While at it, use the same arguments for BUILD_CMDS and
INSTALL_TARGET_CMDS.

Thanks to Yann for the live review during the Buildroot summer camp.

Fixes:
http://autobuild.buildroot.net/results/364/36470db2c262d2e1fda5144a08cfe221831e093e

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Petr Kulhavy <brain@jikos.cz>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/linuxptp/linuxptp.mk

@@ -13,8 +13,9 @@ LINUXPTP_LICENSE_FILES = COPYING
 define LINUXPTP_BUILD_CMDS
 	$(TARGET_MAKE_ENV) \
 	CROSS_COMPILE="$(TARGET_CROSS)" \
+	KBUILD_OUTPUT=$(STAGING_DIR) \
 	$(MAKE) \
-		KBUILD_OUTPUT=$(TARGET_DIR) \
+		prefix=/usr \
 		EXTRA_CFLAGS="$(TARGET_CFLAGS)" \
 		EXTRA_LDFLAGS="$(TARGET_LDFLAGS)" \
 		-C $(@D) all
@@ -22,10 +23,13 @@ endef
 
 define LINUXPTP_INSTALL_TARGET_CMDS
 	$(TARGET_MAKE_ENV) \
+	CROSS_COMPILE="$(TARGET_CROSS)" \
+	KBUILD_OUTPUT=$(STAGING_DIR) \
 	$(MAKE) \
 		prefix=/usr \
 		DESTDIR=$(TARGET_DIR) \
-		$(TARGET_CONFIGURE_OPTS) \
+		EXTRA_CFLAGS="$(TARGET_CFLAGS)" \
+		EXTRA_LDFLAGS="$(TARGET_LDFLAGS)" \
 		-C $(@D) install
 
 	$(INSTALL) -D -m 644 $(LINUXPTP_PKGDIR)/linuxptp.cfg \