esp_light_sleep_start() will stall the other CPU via esp_ipc_isr_stall_other_cpu(). After stalling the other CPU,
will call esp_clk_... API which themselves take locks. If the other stalled CPU is holding those locks, this will
result in a deadlock.
This commit adds a workaround calling esp_clk_private_lock() to take the lock before stalling the other CPU.
esp_clk used to be protected using _lock_t (i.e., a FreeRTOS Mutex). However, esp_clk API is current called from
from critical sections, thus mutex should not be used (as they can be blocking).
This commit updates esp_clk to use spinlocks for critical sections instead.
Note: Added a small fix to exclude esp_clk.c from bootloader builds in the
legacy build system (this is already the case in CMake).
