env_mmc: fix buffer allocation for armv7

commit d196bd88 adds
redundand environment to mmc. The usage of malloc in
env_relocate_spec triggers cache errors on armv7.

Tested on a not mainlined i.MX53 board:

Board: TQMa53
I2C:   ready
DRAM:  512 MiB
MMC:   FSL_SDHC: 0, FSL_SDHC: 1
ERROR: v7_dcache_inval_range - start address is not aligned - 0x8f57c2d8
ERROR: v7_dcache_inval_range - stop address is not aligned - 0x8f57e2d8
ERROR: v7_dcache_inval_range - start address is not aligned - 0x8f57e2e0
ERROR: v7_dcache_inval_range - stop address is not aligned - 0x8f5802e0
Using default environment

Signed-off-by: Markus Niebel <Markus.Niebel@tqs.de>