Add USB feature for flashcart (#85)

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## Description
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Adds a feature to the flashcart to determine whether USB is supported.

## Motivation and Context
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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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## Types of changes
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- [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>

Co-authored-by: Mateusz Faderewski <sc@mateuszfaderewski.pl>

src/flashcart/64drive/64drive.c

@@ -74,6 +74,7 @@ static bool d64_has_feature (flashcart_features_t feature) {
     switch (feature) {
         case FLASHCART_FEATURE_64DD: return false;
         case FLASHCART_FEATURE_RTC: return true;
+        case FLASHCART_FEATURE_USB: return true;
         default: return false;
     }
 }

src/flashcart/flashcart.h

@@ -28,6 +28,7 @@ typedef enum {
 typedef enum {
     FLASHCART_FEATURE_64DD,
     FLASHCART_FEATURE_RTC,
+    FLASHCART_FEATURE_USB,
 } flashcart_features_t;
 
 /** @brief Flashcart save type enumeration */

src/flashcart/sc64/sc64.c

@@ -261,6 +261,7 @@ static bool sc64_has_feature (flashcart_features_t feature) {
     switch (feature) {
         case FLASHCART_FEATURE_64DD: return true;
         case FLASHCART_FEATURE_RTC: return true;
+        case FLASHCART_FEATURE_USB: return true;
         default: return false;
     }
 }