env_mmc: Fix crashing bug encountered after enabling ARM relocation

The crash was occuring in env_relocate because it was being called prior
to mmc_initialize.  This patch moves the MMC initialization earlier in
the init process.

This patch also cleans up the env_relocate_spec code in env_mmc.c

Signed-off-by: Steve Sakoman <steve.sakoman@linaro.org>
Acked-by: Stefano Babic <sbabic@denx.de>