EGL: Use cache sizes defined in the BoardConfig

This change introduces two new BoardConfig variables to control the size
limits of the EGL blob cache.  MAX_EGL_CACHE_ENTRY_SIZE is the size
limit for values inserted into the cache, and MAX_EGL_CACHE_SIZE is the
size limit for all entries in the entire cache (including both keys and
values).  If either of these BoardConfig variables are not defined then
a default size limit is used instead.

Change-Id: I6703d93f966b6389c6499f23d841e42339f9c9d7