Faster timer option for atsam4l + fix race condition in previous atsam4l timer driver

miosix/arch/common/core/stm32f1_l1_os_timer.cpp

miosix/arch/common/drivers/serial_atsam4l.cpp

miosix/arch/cortexM4_atsam4l/common/drivers/clock.cpp

miosix/arch/cortexM4_atsam4l/common/drivers/clock.h

...arch/cortexM4_atsam4l/common/interfaces-impl/os_timer.cpp

...rch/cortexM4_atsam4l/atsam4lc2aa_generic/board_settings.h

miosix/interfaces/os_timer.h