u-boot: Disable environment storage

U-Boot can be configured via environment variables which can be saved to
various storage devices. This usually defaults to MMC or SPI depending
on where it boots from, but assumes the device's layout is controlled by
U-Boot.

We should store the environment in SPI flash, but we also need to
configure coreboot FMAPs to reserve the area U-Boot would use as its
environment storage. For now, disable environment storage by setting
ENV_IS_NOWHERE=y to avoid overwriting random regions of SPI or MMC if
someone tries to save the variables.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>