Add filesystem browsing and enrollment
Add filesystem browsing and enrollment

Add a basic menu system and file explorer. This makes it possible for the
user to enrol keys from media from within shim rather than having to boot
an OS first. This would permit vendors to distribute a signed shim without
having to install their own keys first - the keys could be stored on the
install media instead.

rhinstaller/shim,CyanogenMod/android_external_uefi_shim,android-ia/platform_external_uefi_shim,rhinstaller/shim,CyanogenMod/android_external_uefi_shim,CyanogenMod/android_external_uefi_shim,rhinstaller/shim,android-ia/platform_external_uefi_shim,android-ia/platform_external_uefi_shim