fastboot: ARM: OMAP5: Enable reboot-bootloader

Implemented fb_set_reboot_flag() for OMAP5 to set
an environment variable 'dofastboot' when reboot-bootloader called.

This environment variable will be checked in boot command and fastboot
will be called if the variable is set.
If the bootcmd env variable of OMAP5 common is overwritten with board-specific
command, then these changes will not apply.

This was originally intended for DRA7 platform, but now applies to all OMAP5.

Ref:
http://git.omapzoom.org/?p=repo/u-boot.git;a=commit;h=19da2e436e9806259cf1f4988b9e046ab256bf2c



Signed-off-by: Angela Stegmaier <angelabaker@ti.com>
Signed-off-by: Dileep Katta <dileep.katta@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
[trini: Make it check for !CONFIG_ENV_IS_NOWHERE as we can't saveenv()
in that case]
Signed-off-by: Tom Rini <trini@konsulko.com>