Core: Replace BOOT_MIOS with an explicit "is MIOS" flag

I didn't know better back then, but the boot type is only supposed to
be used for the actual boot params. It shouldn't be used or changed
after booting.

Source/Core/Core/ConfigManager.h

@@ -129,6 +129,7 @@ struct SConfig : NonCopyable
   bool bOverrideGCLanguage = false;
 
   bool bWii = false;
+  bool m_is_mios = false;
 
   // Interface settings
   bool bConfirmStop = false;
@@ -195,7 +196,6 @@ struct SConfig : NonCopyable
     BOOT_ELF,
     BOOT_DOL,
     BOOT_WII_NAND,
-    BOOT_MIOS,
     BOOT_BS2,
     BOOT_DFF
   };

Source/Core/Core/HW/EXI/EXI_DeviceIPL.cpp

@@ -236,9 +236,7 @@ void CEXIIPL::SetCS(int _iCS)
 void CEXIIPL::UpdateRTC()
 {
   u32 epoch =
-      (SConfig::GetInstance().bWii || SConfig::GetInstance().m_BootType == SConfig::BOOT_MIOS) ?
-          WII_EPOCH :
-          GC_EPOCH;
+      (SConfig::GetInstance().bWii || SConfig::GetInstance().m_is_mios) ? WII_EPOCH : GC_EPOCH;
   u32 rtc = Common::swap32(GetEmulatedTime(epoch));
   std::memcpy(m_RTC, &rtc, sizeof(u32));
 }

Source/Core/Core/IOS/MIOS.cpp

@@ -168,7 +168,7 @@ bool Load()
 
   Memory::Write_U32(0x00000000, ADDRESS_INIT_SEMAPHORE);
   NOTICE_LOG(IOS, "IPL ready.");
-  SConfig::GetInstance().m_BootType = SConfig::BOOT_MIOS;
+  SConfig::GetInstance().m_is_mios = true;
   DVDInterface::UpdateRunningGameMetadata();
   return true;
 }

Source/Core/Core/State.cpp

@@ -156,8 +156,7 @@ static std::string DoState(PointerWrap& p)
     return version_created_by;
   }
 
-  bool is_wii =
-      SConfig::GetInstance().bWii || SConfig::GetInstance().m_BootType == SConfig::BOOT_MIOS;
+  bool is_wii = SConfig::GetInstance().bWii || SConfig::GetInstance().m_is_mios;
   const bool is_wii_currently = is_wii;
   p.Do(is_wii);
   if (is_wii != is_wii_currently)