fix(examples): idf_as_lib move flash_ops.c to spi_flash stub library

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.

examples/build_system/cmake/idf_as_lib/stubs/esp32/CMakeLists.txt

@@ -1,4 +1,4 @@
-add_library(stub_esp32 STATIC system_api.c flash_ops.c cpu_start.c)
+add_library(stub_esp32 STATIC system_api.c cpu_start.c)
 target_include_directories(stub_esp32 PUBLIC .)
 add_library(stub::esp32 ALIAS stub_esp32)
 

examples/build_system/cmake/idf_as_lib/stubs/spi_flash/CMakeLists.txt

@@ -1,3 +1,3 @@
-add_library(stub_spi_flash INTERFACE)
+add_library(stub_spi_flash STATIC flash_ops.c)
 target_include_directories(stub_spi_flash INTERFACE .)
 add_library(stub::spi_flash ALIAS stub_spi_flash)