build: Set board as generic if not defined

Observed issue using cmake:

    CMake Error at CMakeLists.txt:136 (string):
    string no output variable specified

This will help to keep iotjs in debian repo.

Change-Id: I2991d324b887e340cb676f7de8ea0dda2ea7c050
Forwarded: jerryscript-project#1943
Bug: jerryscript-project#1945
Bug-Debian: https://bugs.debian.org/957364
Origin: https://github.com/TizenTeam/iotjs/tree/sandbox/rzr/build/review/master
IoT.js-DCO-1.0-Signed-off-by: Philippe Coval rzr@users.sf.net

CMakeLists.txt

@@ -132,6 +132,9 @@ else()
 endif()
 
 # Add board-dependant flags
+if(NOT DEFINED TARGET_BOARD)
+  set(TARGET_BOARD "generic")
+endif()
 iotjs_add_compile_flags(-DTARGET_BOARD=${TARGET_BOARD})
 string(TOUPPER ${TARGET_BOARD} TARGET_BOARD_UPPER)
 string(CONCAT TARGET_BOARD_SYMBOL "TARGET_BOARD_" ${TARGET_BOARD_UPPER})