bootloader: add definition of esp_clk_apb_freq

Commit 8cd04c80 has added a dependency of efuse component on
esp_clk_apb_freq, however there was no definition of this function in
the bootloader context.

Reported at https://esp32.com/viewtopic.php?f=13&t=12035

components/bootloader_support/src/bootloader_clock.c

@@ -58,3 +58,12 @@ void bootloader_clock_configure()
     }
 #endif
 }
+
+#ifdef BOOTLOADER_BUILD
+
+int esp_clk_apb_freq(void)
+{
+    return rtc_clk_apb_freq_get();
+}
+
+#endif // BOOTLOADER_BUILD