Fix #2214, add abstract documentation dependency target

Adds a top level target called "doc-prebuild" which in turn can be made
to depend on any other files that need to exist before documentation can
be built.

cmake/mission_build.cmake

@@ -180,6 +180,7 @@ function(prepare)
  add_custom_target(mission-install COMMAND $(MAKE) install)
  add_custom_target(mission-clean COMMAND $(MAKE) clean)
  add_custom_target(mission-prebuild)
+ add_custom_target(doc-prebuild)
 
  # Locate the source location for all the apps found within the target file
  # This is done by searching through the list of paths to find a matching name
@@ -325,8 +326,8 @@ function(prepare)
  add_subdirectory(${osal_MISSION_DIR} osal_public_api)
  add_subdirectory(${osal_MISSION_DIR}/docs/src ${CMAKE_BINARY_DIR}/docs/osal-apiguide)
 
- add_dependencies(cfe-usersguide osal_public_api_headerlist)
- add_dependencies(mission-doc osal_public_api_headerlist)
+ add_dependencies(cfe-usersguide doc-prebuild)
+ add_dependencies(mission-doc doc-prebuild)
 
  # Pull in any application-specific mission-scope configuration
  # This may include user configuration files such as cfe_mission_cfg.h,
@@ -484,5 +485,3 @@ function(process_arch TARGETSYSTEM)
  add_dependencies(mission-install ${TARGETSYSTEM}-install)
 
 endfunction(process_arch TARGETSYSTEM)
-
-