hwconfig: Utilize getenv_f before relocation to allow for larger buffer

Since we use hwconfig in cases before relocation (like getting DDR
params on FSL PPC systems), we can have strings that exceed the early
small (32 byte) buffer size that getenv will handle.

So we explicitly allocate our own buffer on the stack and use if to
handle getting the hwconfig env string.  We currently utilize a string
length of 128 bytes.

This allows us to get rid of boot messages like:

env_buf too small [32]

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>