app_update: Fix case when elf file SHA256 should be printed by panic handler while cache is disabled

Closes: IDF-1342

components/app_update/Kconfig.projbuild

@@ -22,4 +22,14 @@ menu "Application manager"
             The PROJECT_NAME variable from the build system will not affect the firmware image.
             This value will not be contained in the esp_app_desc structure.
 
+    config APP_RETRIEVE_LEN_ELF_SHA
+        int "The length of APP ELF SHA is stored in RAM(chars)"
+        default 16
+        range 8 64
+        help
+            At startup, the app will read this many hex characters from the embedded APP ELF SHA-256 hash value
+            and store it in static RAM. This ensures the app ELF SHA-256 value is always available
+            if it needs to be printed by the panic handler code.
+            Changing this value will change the size of a static buffer, in bytes.
+
 endmenu # "Application manager"

components/app_update/esp_app_desc.c

@@ -72,13 +72,35 @@ static inline char IRAM_ATTR to_hex_digit(unsigned val)
     return (val < 10) ? ('0' + val) : ('a' + val - 10);
 }
 
+__attribute__((constructor)) void esp_ota_init_app_elf_sha256(void)
+{
+    esp_ota_get_app_elf_sha256(NULL, 0);
+}
+
+/* The esp_app_desc.app_elf_sha256 should be possible to print in panic handler during cache is disabled.
+ * But because the cache is disabled the reading esp_app_desc.app_elf_sha256 is not right and
+ * can lead to a complete lock-up of the CPU.
+ * For this reason we do a reading of esp_app_desc.app_elf_sha256 while start up in esp_ota_init_app_elf_sha256() 
+ * and keep it in the static s_app_elf_sha256 value.
+ */
 int IRAM_ATTR esp_ota_get_app_elf_sha256(char* dst, size_t size)
 {
-    size_t n = MIN((size - 1) / 2, sizeof(esp_app_desc.app_elf_sha256));
-    const uint8_t* src = esp_app_desc.app_elf_sha256;
+    static char s_app_elf_sha256[CONFIG_APP_RETRIEVE_LEN_ELF_SHA / 2];
+    static bool first_call = true;
+    if (first_call) {
+        first_call = false;
+        const uint8_t* src = esp_app_desc.app_elf_sha256;
+        for (size_t i = 0; i < sizeof(s_app_elf_sha256); ++i) {
+            s_app_elf_sha256[i] = src[i];
+        }
+    }
+    if (dst == NULL || size == 0) {
+        return 0;
+    }
+    size_t n = MIN((size - 1) / 2, sizeof(s_app_elf_sha256));
     for (size_t i = 0; i < n; ++i) {
-        dst[2*i] = to_hex_digit(src[i] >> 4);
-        dst[2*i + 1] = to_hex_digit(src[i] & 0xf);
+        dst[2*i] = to_hex_digit(s_app_elf_sha256[i] >> 4);
+        dst[2*i + 1] = to_hex_digit(s_app_elf_sha256[i] & 0xf);
     }
     dst[2*n] = 0;
     return 2*n + 1;