Previously the loop in esp_timer_impl_set_alarm was necessary to catch
the case when timer count wraps around (goes from 2^32 - 1 to 0).
Since ALARM_OVERFLOW_VAL was reduced from 2^32 - 1 to 0xefffffff,
this is no longer necessary.
Fixes https://github.com/espressif/esp-idf/issues/1891
Moves the ets_timer_arm() / ets_timer_disarm() code paths to RAM
Overhead is 740 bytes of IRAM, 0 bytes DRAM
(For comparison: If all of esp_timer.c is moved to RAM, overhead is 1068 bytes IRAM and 480 bytes DRAM.)
The implementation of esp_timer_get_time used a critical section, which
resulted in a call time of ~1.8us. To make esp_timer_get_time more
useable as a high-resolution time source, this change replaces the lock
with polling. Call time is reduced to ~0.7us.
