Fix small data area in case no fixed size is desired. Rename
bsp_section_set_sdata_sbss_size into bsp_section_small_data_area_size
since this symbol reflects the overall small data area size (including
space for libdl). Do not use bsp_section_sbss_size before definition in
linker command file. Add new symbols to <bsp/linker-symbols.h>.
Update #3687.