Add RTC feature for flashcart (#58)

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## Description
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Adds a feature to the flashcart to determine whether RTC is supported.
This is at minimum required for correct handling of what to show on the
main menu.
It also will help with a future context menu to set the RTC time.

## Motivation and Context
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<!--- If it fixes/closes/resolves an open issue, please link to the
issue here -->
Some flashcarts (ED64 X5, V2.5, V2.0, V1.0 & ED64P ) do not support it.

## How Has This Been Tested?
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## Screenshots
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## Types of changes
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the boxes that apply: -->
- [x] Improvement (non-breaking change that adds a new feature)
- [ ] Bug fix (fixes an issue)
- [ ] Breaking change (breaking change)
- [ ] Config and build (change in the configuration and build system,
has no impact on code or features)

## Checklist:
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- [ ] My code follows the code style of this project.
- [ ] My change requires a change to the documentation.
- [ ] I have updated the documentation accordingly.
- [ ] I have added tests to cover my changes.
- [ ] All new and existing tests passed.

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Signed-off-by: GITHUB_USER <GITHUB_USER_EMAIL>

src/flashcart/64drive/64drive.c

@@ -73,6 +73,7 @@ static flashcart_err_t d64_deinit (void) {
 static bool d64_has_feature (flashcart_features_t feature) {
     switch (feature) {
         case FLASHCART_FEATURE_64DD: return false;
+        case FLASHCART_FEATURE_RTC: return true;
         default: return false;
     }
 }

src/flashcart/flashcart.h

@@ -27,6 +27,7 @@ typedef enum {
 /** @brief List of optional supported flashcart features */
 typedef enum {
     FLASHCART_FEATURE_64DD,
+    FLASHCART_FEATURE_RTC,
 } flashcart_features_t;
 
 /** @brief Flashcart save type enumeration */

src/flashcart/sc64/sc64.c

@@ -260,6 +260,7 @@ static flashcart_err_t sc64_deinit (void) {
 static bool sc64_has_feature (flashcart_features_t feature) {
     switch (feature) {
         case FLASHCART_FEATURE_64DD: return true;
+        case FLASHCART_FEATURE_RTC: return true;
         default: return false;
     }
 }