MdeModulePkg:Update Boot Manager form when enter it.

MdeModulePkg:Update Boot Manager form when enter it.

Currently BootManager is an independent library,used by UiApp.It only load boot
option when enter UiApp.So when add boot option through boot maintenance manager,
it will not display in boot manager. In order to fix this issue,now we update the
boot manager form every time we enter it.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19440 6f19259b-4bc3-4df7-8a09-765794883524