This commit updates the esp_cpu.h API. The new API presents a new
abstraction of the CPU where CPU presents the following interfaces:
- CPU Control (to stall/unstall/reset the CPU)
- CPU Registers (to read registers commonly used in SW such as SP, PC)
- CPU Interrupts (to inquire/allocate/control the CPUs 32 interrupts)
- Memory Port (to configure the CPU's memory bus for memory protection)
- Debugging (to configure/control the CPU's debugging port)
Note: Also added FORCE_INLINE_ATTR to the DoxyFile in order to pass doc
builds for esp_cpu.h
External memory is accessed via SPI0. When modifying the SPI0 registers,
should put the code in internal RAM. Otherwise when there is an ongoing
SPI0 transaction, CPU changes the SPI0 registers. This is dangerous.
Besides, modifying SPI0 registers may lead external memory to an
unstable state. Therefore putting these code in internal RAM is
necessary.
