vendor: Don't use absolute dir for mkbootimg symlink

I noticed that the absolute symlink did not work on my system when
trying to execute the script inside it's installation directory before
the package is installed.
While investigating I noticed that on SUSE the symlink gets relinked
to be relative to be relative to the install location of the symlink
after which the link works.
Using a relative link seems safer to me.

Signed-off-by: Björn Bidar <bjorn.bidar@jolla.com>

vendor/CMakeLists.mkbootimg.txt

@@ -1,7 +1,7 @@
-set(MKBOOTIMG_SCRIPTS_DIR ${CMAKE_INSTALL_FULL_DATADIR}/android-tools/mkbootimg)
+set(MKBOOTIMG_SCRIPTS_DIR ${CMAKE_INSTALL_DATADIR}/android-tools/mkbootimg)
 install(PROGRAMS mkbootimg/mkbootimg.py DESTINATION ${MKBOOTIMG_SCRIPTS_DIR})
 add_custom_target(mkbootimg_symlink ALL COMMAND ${CMAKE_COMMAND} -E create_symlink
-	${MKBOOTIMG_SCRIPTS_DIR}/mkbootimg.py
+	../${MKBOOTIMG_SCRIPTS_DIR}/mkbootimg.py
 	${CMAKE_CURRENT_BINARY_DIR}/mkbootimg)
 install(FILES ${CMAKE_CURRENT_BINARY_DIR}/mkbootimg DESTINATION ${CMAKE_INSTALL_BINDIR})
 install(FILES mkbootimg/gki/generate_gki_certificate.py DESTINATION ${MKBOOTIMG_SCRIPTS_DIR}/gki)