add pico_multi_booter code

Code/pico_multi_booter/README.md

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+# PicoCalc multi booter
+
+Here is a bootloader for PicoCalc combined slightly modified [PicoMite](https://github.com/madcock/PicoMiteAllVersions) and [SD boot](https://github.com/adwuard/Picocalc_SD_Boot)   
+
+- Pico1 
+- No sdcard inserted ,PicoMite will show up.  
+- Sdcard inserted, SD boot menu will show up, load third pico app bin to run at FLASH TARGET OFFSET 2048k-940k  
+
+## How to compile
+```
+export PICO_SDK_PATH=/where/picosdk/is
+mkdir build
+cd build
+cmake ..
+make
+```
+## How to run 
+
+Just copy **combined.uf2** into PicoCalc
+
+
+## PicoMite
+configuration.h
+```
+#define FLASH_TARGET_OFFSET (920 * 1024)
+```
+
+## sd_boot
+
+config.h
+```
+#define SD_BOOT_FLASH_OFFSET         (940 * 1024)
+```
+
+### SD Card Application Build and Deployment
+**Important Note:**   
+```
+Applications intended for SD card boot "MUST REBUILD" using a custom linker script to accommodate the program's offset(940k) address.
+
+Applications intended for SD card boot is in **bin** format, not uf2.
+ 
+```
+
+--- 
+This section explains how to build and deploy applications on an SD card. Below is a simple example of a CMakeLists.txt for an application.
+
+
+#### Step 1 Copy Custom Link Script
+Copy `memmap_sdcard_app.ld` to your project repository.
+
+
+#### Step 2 Add Custom Link Script to CMakeList.txt
+In the project for multi boot, add custom link script to CMakeList.txt,usually at bottom.   
+We also need to use `pico_add_extra_outputs` to sure that there will .bin file generated.   
+
+```
+...
+pico_add_extra_outputs(${CMAKE_PROJECT_NAME})
+...
+
+function(enable_sdcard_app target)
+  #pico_set_linker_script(${target} ${CMAKE_SOURCE_DIR}/memmap_sdcard_app.ld)
+  if(${PICO_PLATFORM} STREQUAL "rp2040")
+    pico_set_linker_script(${CMAKE_PROJECT_NAME} ${CMAKE_SOURCE_DIR}/memmap_default_rp2040.ld)
+  elseif(${PICO_PLATFORM} MATCHES "rp2350")
+    pico_set_linker_script(${CMAKE_PROJECT_NAME} ${CMAKE_SOURCE_DIR}/memmap_default_rp2350.ld)
+  endif()
+endfunction()
+
+enable_sdcard_app(${CMAKE_PROJECT_NAME})
+```
+#### Build and Deployment Process
+1. Build the project using the above CMakeLists.txt.
+
+```bash
+mkdir build; 
+cd build
+export PICO_SDK_PATH=/path/to/pico-sdk 
+cmake ..
+make
+```
+
+#### Step 3 Your Custom Application Is Ready For SD Card Boot 
+Once the build is complete, copy the generated `.bin` file to the `/sd` directory of the SD card.  
+
+
+