eigen: follow default installation scheme

eigen's build-system (cmake), by default, installs the header files
in /usr/include/eigen3

Signed-off-by: Davide Viti <d.viti@infosolution.it>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/eigen/eigen.mk

@@ -12,12 +12,14 @@ EIGEN_LICENSE = MPL2, BSD-3c, LGPLv2.1
 EIGEN_LICENSE_FILES = COPYING.MPL2 COPYING.BSD COPYING.LGPL COPYING.README
 EIGEN_INSTALL_STAGING = YES
 EIGEN_INSTALL_TARGET = NO
+EIGEN_DEST_DIR = $(STAGING_DIR)/usr/include/eigen3
 
 # This package only consists of headers that need to be
 # copied over to the sysroot for compile time use
 define EIGEN_INSTALL_STAGING_CMDS
-	$(RM) -r $(STAGING_DIR)/usr/include/Eigen
-	cp -a $(@D)/Eigen $(STAGING_DIR)/usr/include/
+	$(RM) -r $(EIGEN_DEST_DIR)
+	mkdir -p $(EIGEN_DEST_DIR)
+	cp -a $(@D)/Eigen $(EIGEN_DEST_DIR)
 endef
 
 $(eval $(generic-package))