image_types_reach.bbclass: fix $SDCARD_GENERATION_COMMAND for g2s and g2h

SDCARD_GENERATION_COMMAND for g2s and g2h is configured with
generate_g2sh_sdcard, however, the defined function was
generate_g2hs_sdcard.  To fix that, we change the function name (in
its definition) to the name pointed by SDCARD_GENERATION_COMMAND
(generate_g2sh_sdcard).

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>