Added ability to specify additional search path

I wanted to use a different location for my install prefix. Doing so broke FindCMSIS.cmake

From what I could tell, the problem comes from the gcc_stm32.cmake

SET(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
SET(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)

I changed the definition of CMAKE_FIND_ROOT_PATH to include an optional variable EXTRA_FIND_PATH

If this variable is not set, the normal behavior is set. If it is set, this path is included when using find_path/find_library.

Additionally, I changed the searching of the CMSIS_LINKER_SCRIPT to use FIND_PATH with specified PATH_SUFFIXES instead of FIND_FILE, to follow the convention defined by the search for CMSIS_INCLUDE_DIR