Use only one backup register for CMD_BOOT for stm32f103 based targets

This is behavior is more akin to what most STM32F103xx applications wanting to
enter the bootloader are accustomed to. Should retain backwards compatibility
with existing applications writing to both registers, the higher one just won't
be checked by the bootloader anymore.

src/stm32f103/backup.c

@@ -24,18 +24,15 @@
 
 #define RTC_BKP_DR(reg)  MMIO16(BACKUP_REGS_BASE + 4 + (4 * (reg)))
 
-void backup_write(enum BackupRegister reg, uint32_t value) {
+void backup_write(enum BackupRegister reg, uint16_t value) {
     rcc_periph_clock_enable(RCC_PWR);
     rcc_periph_clock_enable(RCC_BKP);
 
     pwr_disable_backup_domain_write_protect();
-    RTC_BKP_DR((int)reg) = value & 0xFFFFUL;
-    RTC_BKP_DR((int)reg+1) = (value & 0xFFFF0000UL) >> 16;
+    RTC_BKP_DR((int)reg) = value;
     pwr_enable_backup_domain_write_protect();
 }
 
-uint32_t backup_read(enum BackupRegister reg) {
-    uint32_t value = (uint32_t)RTC_BKP_DR((int)reg)
-                   | ((uint32_t)RTC_BKP_DR((int)reg+1) << 16);
-    return value;
+uint16_t backup_read(enum BackupRegister reg) {
+    return RTC_BKP_DR((int)reg);
 }

src/stm32f103/backup.h

@@ -32,7 +32,7 @@ enum BackupRegister {
     BKP10,
 };
 
-extern void backup_write(enum BackupRegister reg, uint32_t value);
-extern uint32_t backup_read(enum BackupRegister reg);
+extern void backup_write(enum BackupRegister reg, uint16_t value);
+extern uint16_t backup_read(enum BackupRegister reg);
 
 #endif

src/stm32f103/target_stm32f103.c

@@ -60,7 +60,7 @@ _Static_assert((FLASH_BASE + FLASH_SIZE_OVERRIDE >= APP_BASE_ADDRESS),
 #endif
 
 #ifndef CMD_BOOT
-#define CMD_BOOT 0x544F4F42UL
+#define CMD_BOOT 0x4F42UL
 #endif
 
 void target_clock_setup(void) {
@@ -166,7 +166,7 @@ const usbd_driver* target_usb_init(void) {
 bool target_get_force_bootloader(void) {
     bool force = false;
     /* Check the RTC backup register */
-    uint32_t cmd = backup_read(REG_BOOT);
+    uint16_t cmd = backup_read(REG_BOOT);
     if (cmd == CMD_BOOT) {
         force = true;
     }