path issue when loading Super Game Boy

when Super Game Boy bios is located in , it will not find it and gives error like this:

RetroArch [libretro INFO] :: [Memory]: ID 41, Request "sgb.boot.rom".
RetroArch [libretro INFO] :: Load BIOS.
RetroArch [INFO] :: Environ SYSTEM_DIRECTORY: ".\system".
RetroArch [libretro ERROR] :: Cannot find requested file in system directory: ".\system/sgb.boot.rom
".

Syntax used:

`E:\EMULATORS\retroarch\retroarch.exe -L "E:\EMULATORS\retroarch\cores\bsnes_balanced_libretro.dll" "C:\Users\MEDIA\Desktop\New folder (2)\Super Game Boy (World).sfc" --subsystem sgb "C:\Users\MEDIA\Desktop\New folder (2)\Castlevania Legends (USA, Europe).gb"
`

moving the bios file into the Super Game Boy rom or into the gameboy rom(or what ever rom you are trying to load SuperGameBoy with) works.

this also affects bsnes-mercury

forum discussion: https://forums.libretro.com/t/super-game-boy-enough-is-enough/10872/5

Possible problematic area: https://github.com/libretro/bsnes-libretro/blob/libretro/target-libretro/libretro.cpp#L207

Tested under Windows 7 64bit.

Start at target-libretro/libretro.cpp around line 207 and reproduce the Windows command shown in the issue with the BIOS in the system directory. Trace how the system directory path is assembled for the Super Game Boy subsystem. Done means sgb.boot.rom is found in for bsnes2014 and the related bsnes-mercury case is addressed.