[partition] Introduce check for EFI partition type-and-size

Re-use the existing message about partition type and size,
since I don't want to introduce another message with all the
specifics; give a works-always message instead.

The check itself is also straightforward, avoiding all of the
nuances and technically-this-might-work cases: FAT32, 300MiB+.

FIXES #607

src/modules/partition/PartitionViewStep.cpp

@@ -445,7 +445,7 @@ PartitionViewStep::onLeave()
 
             QString message;
             QString description;
-            if ( !esp )
+            if ( !esp || ( esp && !PartUtils::isEfiFilesystemSuitable( esp ) ) )
             {
                 message = tr( "No EFI system partition configured" );
                 description = tr( "An EFI system partition is necessary to start %1."

src/modules/partition/core/PartUtils.cpp

@@ -446,6 +446,34 @@ isEfiSystem()
     return QDir( "/sys/firmware/efi/efivars" ).exists();
 }
 
+bool
+isEfiFilesystemSuitable(const Partition* candidate)
+{
+    auto type = candidate->fileSystem().type();
+    auto size = candidate->capacity();  // bytes
+
+    using CalamaresUtils::Units::operator""_MiB;
+
+    switch( type )
+    {
+        case FileSystem::Type::Fat32:
+            if ( size >= 300_MiB )
+            {
+                return true;
+            }
+            cWarning() << "FAT32 filesystem is too small (" << size << "bytes)";
+            return false;
+        case FileSystem::Type::Fat12:
+        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
 isEfiBootable( const Partition* candidate )
 {

src/modules/partition/core/PartUtils.h

@@ -82,6 +82,12 @@ OsproberEntryList runOsprober( DeviceModel* dm );
  */
 bool isEfiSystem();
 
+/**
+ * @brief Is the @p partition suitable as an EFI boot partition?
+ * Checks for filesystem type (FAT32) and size (300MiB at least).
+ */
+bool isEfiFilesystemSuitable( const Partition* candidate );
+
 /**
  * @brief Is the given @p partition bootable in EFI? Depending on
  * the partition table layout, this may mean different flags.