MdeModulePkg: Use BmCharToUint in BmIsKeyOptionVariable

The patch also moves the BmCharToUint to BmMisc.c because it
belongs to misc functions.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Sunny Wang <sunnywang@hpe.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18855 6f19259b-4bc3-4df7-8a09-765794883524

MdeModulePkg/Library/UefiBootManagerLib/InternalBm.h

@@ -434,4 +434,17 @@ BmPrintDp (
   EFI_DEVICE_PATH_PROTOCOL            *DevicePath
   );
 
+/**
+  Convert a single character to number.
+  It assumes the input Char is in the scope of L'0' ~ L'9' and L'A' ~ L'F'
+
+  @param    Char   The input char which need to convert to int.
+
+  @return  The converted 8-bit number or (UINTN) -1 if conversion failed.
+**/
+UINTN
+BmCharToUint (
+  IN CHAR16                           Char
+  );
+
 #endif // _INTERNAL_BM_H_