feat(refactor): Restructure project for improved modularity and clarity

This commit introduces a major refactoring of the project structure to align
with Zephyr's recommended multi-application and library organization.

Key changes include:
- Relocation of custom modules from 'software/modules/' to 'software/lib/'.
- Introduction of a central 'software/CMakeLists.txt' to manage application
  and library subdirectories.
- Creation of new Kconfig files for 'software/' and 'software/apps/slave_node/'
  to define project-wide and application-specific configurations.
- Removal of the 'gateway' and 'stm32g431_tests' applications.
- Removal of 'shell_modbus.c' and 'shell_system.c' from 'slave_node' application's
  direct source files, indicating a shift towards library-based shell commands.
- Updates to 'software/apps/slave_node/CMakeLists.txt', 'prj.conf', and
  'boards/bluepill_f103rb.conf' to reflect the new structure and dependencies.

software/lib/fwu/fwu.c

@@ -0,0 +1,45 @@
+#include <zephyr/kernel.h>
+#include <zephyr/sys/crc.h>
+#include <zephyr/sys/byteorder.h>
+#include <zephyr/logging/log.h>
+#include <lib/fwu.h>
+
+LOG_MODULE_REGISTER(fwu, LOG_LEVEL_INF);
+
+#define FWU_BUFFER_SIZE 256
+static uint8_t fwu_buffer[FWU_BUFFER_SIZE];
+static uint32_t fwu_chunk_offset = 0;
+static uint16_t fwu_chunk_size = 0;
+static uint16_t fwu_last_chunk_crc = 0;
+
+void fwu_init(void) {}
+
+void fwu_handler(uint16_t addr, uint16_t reg)
+{
+    // This is a simplified handler. In a real scenario, you would have a proper mapping
+    // between register addresses and actions.
+    if (addr == 0x0100) { // FWU_COMMAND
+        if (reg == 1) { LOG_INF("FWU: Chunk at offset %u (size %u) verified.", fwu_chunk_offset, fwu_chunk_size); }
+        else if (reg == 2) { LOG_INF("FWU: Finalize command received. Rebooting (simulated)."); }
+    } else if (addr == 0x0101) { // FWU_CHUNK_OFFSET_LOW
+        fwu_chunk_offset = (fwu_chunk_offset & 0xFFFF0000) | reg;
+    } else if (addr == 0x0102) { // FWU_CHUNK_OFFSET_HIGH
+        fwu_chunk_offset = (fwu_chunk_offset & 0x0000FFFF) | ((uint32_t)reg << 16);
+    } else if (addr == 0x0103) { // FWU_CHUNK_SIZE
+        fwu_chunk_size = (reg > FWU_BUFFER_SIZE) ? FWU_BUFFER_SIZE : reg;
+    } else if (addr >= 0x0180 && addr < (0x0180 + (FWU_BUFFER_SIZE / 2))) {
+        uint16_t index = (addr - 0x0180) * 2;
+        if (index < sizeof(fwu_buffer)) {
+            sys_put_be16(reg, &fwu_buffer[index]);
+            if (index + 2 >= fwu_chunk_size) {
+                fwu_last_chunk_crc = crc16_ccitt(0xffff, fwu_buffer, fwu_chunk_size);
+                LOG_INF("FWU: Chunk received, CRC is 0x%04X", fwu_last_chunk_crc);
+            }
+        }
+    }
+}
+
+uint16_t fwu_get_last_chunk_crc(void)
+{
+    return fwu_last_chunk_crc;
+}