configs: increase SYS_MALLOC_F_LEN for STM32 MCU's board

Some STM32 MCU's board need their SYS_MALLOC_F_LEN value enlarged
to avoid the "alloc space exhausted" error message during their boot
process.
Use the default SYS_MALLOC_F_LEN value which is set to 0x2000 in
Kconfig.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>