app_update: fix incorrect first byte from esp_ota_get_app_elf_sha256
app_update: fix incorrect first byte from esp_ota_get_app_elf_sha256

At -O2 optimization level, GCC seems to optimize out the copying of the
first byte of the checksum, assuming it is zero. This "miscompilation"
happens because the esp_app_desc struct is declared const, but then modified
post-compilation. Casting to volatile disables the optimization.

Closes: https://github.com/espressif/esp-idf/pull/6389

espressif/esp-idf,espressif/esp-idf,espressif/esp-idf,espressif/esp-idf