[partition] Disentangle questions of suitability of ESP

- split into size, type, flags so the warning message can
  be tailored to what is wrong.

src/modules/partition/core/PartUtils.cpp

@@ -447,31 +447,40 @@ isEfiSystem()
 }
 
 bool
-isEfiFilesystemSuitable(const Partition* candidate)
+isEfiFilesystemSuitableType( const Partition* candidate )
 {
     auto type = candidate->fileSystem().type();
+
+    switch ( type )
+    {
+    case FileSystem::Type::Fat32:
+        return true;
+#ifdef WITH_KPMCORE4API
+    case FileSystem::Type::Fat12:
+#endif
+    case FileSystem::Type::Fat16:
+        cWarning() << "FAT12 and FAT16 are probably not supported by EFI";
+        return false;
+    default:
+        cWarning() << "EFI boot partition must be FAT32";
+        return false;
+    }
+}
+
+bool
+isEfiFilesystemSuitableSize( const Partition* candidate )
+{
     auto size = candidate->capacity();  // bytes
 
     using CalamaresUtils::Units::operator""_MiB;
-
-    switch( type )
+    if ( size >= 300_MiB )
     {
-        case FileSystem::Type::Fat32:
-            if ( size >= 300_MiB )
-            {
-                return true;
-            }
-            cWarning() << "FAT32 filesystem for EFI is too small (" << size << "bytes)";
-            return false;
-#ifdef WITH_KPMCORE4API
-        case FileSystem::Type::Fat12:
-#endif
-        case FileSystem::Type::Fat16:
-            cWarning() << "FAT12 and FAT16 are probably not supported by EFI";
-            return false;
-        default:
-            cWarning() << "EFI boot partition must be FAT32";
-            return false;
+        return true;
+    }
+    else
+    {
+        cWarning() << "Filesystem for EFI is too small (" << size << "bytes)";
+        return false;
     }
 }
 
@@ -508,6 +517,15 @@ isEfiBootable( const Partition* candidate )
 #endif
 }
 
+// TODO: this is configurable via the config file **already**
+size_t
+efiFilesystemMinimumSize()
+{
+    using CalamaresUtils::Units::operator""_MiB;
+    return 300_MiB;
+}
+
+
 QString
 canonicalFilesystemName( const QString& fsName, FileSystem::Type* fsType )
 {

src/modules/partition/core/PartUtils.h

@@ -84,9 +84,24 @@ bool isEfiSystem();
 
 /**
  * @brief Is the @p partition suitable as an EFI boot partition?
- * Checks for filesystem type (FAT32) and size (300MiB at least).
+ * Checks for filesystem type (FAT32).
  */
-bool isEfiFilesystemSuitable( const Partition* candidate );
+bool isEfiFilesystemSuitableType( const Partition* candidate );
+
+/**
+ * @brief Is the @p partition suitable as an EFI boot partition?
+ * Checks for filesystem size (300MiB, see efiFilesystemMinimumSize).
+ */
+bool isEfiFilesystemSuitableSize( const Partition* candidate );
+
+/** @brief Returns the minimum size of an EFI boot partition.
+ *
+ * This is determined as 300MiB, based on the FAT32 standard
+ * and EFI documentation (and not a little discussion in Calamares
+ * issues about what works, what is effective, and what is mandated
+ * by the standard and how all of those are different).
+ */
+size_t efiFilesystemMinimumSize();
 
 /**
  * @brief Is the given @p partition bootable in EFI? Depending on