Update memory allocation from meram_alloc_memory_block()

According to the MERAM spec, the meram memory allocation should be aligned
with the same units as the block size (i.e. a 16KB block must start on
a 16K boundary)
Add meram_get_required_memory_size

With this method, the required MERAM memory size can be calculated
from the buffers stride and required number of MERAM lines.
Merge pull request #1 from dhobsong/master

Pull request for libshmeram