Move common shmem parameter declarations to shmem.h.

boards/atlflight/eagle/qurt-default.cmake

platforms/posix/src/px4_layer/shmem_posix.cpp

platforms/qurt/src/px4_layer/qurt_stubs.c

platforms/qurt/src/px4_layer/shmem_qurt.cpp

src/lib/parameters/parameters_shmem.cpp

src/modules/muorb/adsp/px4muorb.cpp

src/platforms/shmem.h