The Linux build was broken after IDF flash API was updated without updating
the Linux stub library in the example. This commit updates the spi_flash stub
library such that:
- The API now matches same API as IDF's spi_flash component
- Links the stub_esp32 library to pull in basic types and defines
Example linux build of the example demonstrates "esp32" and "spi_flash" stub
libraries (roughly analogous to "esp_system" and "spi_flash" components).
This commit moves the "flash_ops.c" file to the "spi_flash" stub library as it
is a flash related funciton.
Also renamed the header to "esp_flash.h" (in order to match current header name
in IDF). This is a prerequisite to fixing the linux build of this example.
GCC prefixes underscore on symbol names on macOS, and so the set
undefined symbol `-u app_main` won't work. However, this is unecessary,
as the source file that defines it is specified directly to the
executable, and not through a static library.
Closes https://github.com/espressif/esp-idf/issues/4168
