cmake: Fix support for non-standard psp source directory

[jbohren-hbr]

Describe the contribution
This PR fixes a bug which prevents non-standard psp source directory specification via the environment variable $CFS_APP_PATH or the cmake variable ${psp_SEARCH_PATH}.

The cFE cmake build infrastructure uses the environment / cmake variable $CFS_APP_PATH
as well as the cmake meta-variable pattern ${${APP}_SEARCH_PATH} to locate apps and other modules (such as psp), and subsequently sets the variable pattern ${${APP}_MISSION_DIR} (see mission_build.cmake lines 202-222).

However, the resulting variable ${psp_MISSION_DIR} is not used in process_arch() in arch_build.cmake line 588 when including a target platform's build_options.cmake file. This causes a cmake configuration failure when trying to use a non-standard psp location.

Testing performed
Build with psp in non-standard location, with the CFS_APP_PATH environment variable set:

# set up standard cFS bundle per https://github.com/nasa/cFS
git clone --recursive https://github.com/nasa/cFS.git
cd cFS
cp -R cfe/cmake/sample_defs .
cp cfe/cmake/Makefile.sample Makefile
# hide psp
mkdir alt
mv psp alt/psp
# define alternate search path
export CFS_APP_PATH=$(pwd)/alt
# build sample mission
make prep
make

Before this PR:

...

-- Generated cfe_platform_cfg.h from /***REDACTED**/cFS/sample_defs configuration
CMake Error at cmake/arch_build.cmake:588 (include):
  include could not find load file:

    /***REDACTED***/cFS/psp/fsw/pc-linux/make/build_options.cmake
Call Stack (most recent call first):
  CMakeLists.txt:124 (process_arch)


-- Configuring incomplete, errors occurred!

...

After this PR:

...

-- Generated cfe_platform_cfg.h from /***REDACTED***/cFS/sample_defs configuration
-- BSP Selection: generic-linux at /***REDACTED***/cFS/osal/src/bsp/generic-linux
-- OSAL Compile Definitions: _LINUX_OS_;_XOPEN_SOURCE=600
-- OSAL Selection: posix at /***REDACTED**/cFS/osal/src/os/posix
-- PSP Selection: pc-linux

...

Built target mission-all

Expected behavior changes
This should only affect projects which use the environment variable $CFS_APP_PATH or the cmake variable ${psp_SEARCH_PATH}. Currently, they are being ignored for psp's build_options.cmake path resolution, which means some users may be building with different target build options than they think they are based on the intended behavior.

System(s) tested on

• Hardware: PC
• OS: Ubuntu 18.04
• Versions: cFE v6.8.0-rc1+dev933

Additional context

Aside, this is also suggested as a method to specify a non-standard psp source directory in mission_defaults.cmake line 67 since PR #751 with the following:

set(psp_SEARCH_PATH ".")

However, this will lead to the same cmake configure error as above without this fix.

Third party code
No thirdy party code included in this contribution.

Contributor Info - All information REQUIRED for consideration of pull request
Jonathan Bohren, Honeybee Robotics

[astrogeco]

Thanks for the contribution @jbohren-hbr ! I opened a related issue to help us write the version description document. Can you double check it and add comments as needed?
#1921

[jbohren-hbr]

#1921 looks good to me!

[astrogeco]

CCB:2021-09-01 APPROVED