bootm: fixup silent Linux out of BOOTM_STATE_LOADOS state

The function fixup_silent_linux() is called in status BOOTM_STATE_LOADOS
to silence Linux if variable 'silent' is set.
Currently only the 'bootm' command state machine contains
BOOTM_STATE_LOADOS, but others like 'booti' or 'bootz' commands do not.
This means silent Linux does not work with these commands.

This patch moves the fixup_silent_linux() call out of the
BOOTM_STATE_LOADOS state and into BOOTM_STATE_OS_PREP, to silence Linux
independently of the used command (booti, bootm or bootz).

Signed-off-by: Hector Palacios <hector.palacios@digi.com>