esp32: Add support for noinit variables in SPIRAM

Add Kconfig option SPIRAM_ALLOW_NOINIT_EXTERNAL_MEMORY
When enabled, a new linker script rule (from esp32.extram.noinit.ld)
places any variables in the .ext_ram.noinit section in SPIRAM.

This section is exempted from the startup SPIRAM memory test and is
not zero-initialized or added to the malloc pool, making it usable
for noinit variables that persist across reset.

The EXT_RAM_NOINIT_ATTR macro places variables in this section.

components/esp_system/ld/esp32/esp32.extram.noinit.ld

@@ -0,0 +1,14 @@
+/* This section is only included if CONFIG_SPIRAM_ALLOW_NOINIT_EXTERNAL_MEMORY
+   is set, to link some NOINIT sections in PSRAM */
+
+SECTIONS
+{
+   /* external memory bss, from any global variable with EXT_RAM_NOINIT_ATTR attribute*/
+  .ext_ram.noinit (NOLOAD) :
+  {
+    _ext_ram_noinit_start = ABSOLUTE(.);
+    *(.ext_ram.noinit*)
+    . = ALIGN(4);
+    _ext_ram_noinit_end = ABSOLUTE(.);
+  } > extern_ram_seg
+}