Build: Fix CMSIS path for local repo

Fixes the path used for installing CMSIS files so that it works
correctly when a local CMSIS_PATH is supplied rather than using the
automatic clone.

Signed-off-by: Jamie Fox <jamie.fox@arm.com>
Change-Id: I05ff769f29ef3cc0eb7613acfd31bda1fc0e4ceb
diff --git a/cmake/install.cmake b/cmake/install.cmake
index 3573f4e..c9530a8 100644
--- a/cmake/install.cmake
+++ b/cmake/install.cmake
@@ -227,7 +227,7 @@
 
 install(CODE "MESSAGE(\"----- Installing platform NS -----\")")
 
-install(DIRECTORY   ${CMAKE_BINARY_DIR}/lib/ext/cmsis-src/CMSIS/Core/Include
+install(DIRECTORY   $<BUILD_INTERFACE:${CMSIS_PATH}/CMSIS/Core/Include>
         DESTINATION ${INSTALL_PLATFORM_NS_DIR}/ext/cmsis)
 
 if(PLATFORM_DEFAULT_UART_STDOUT)