cfs-coffee: Fix build warning
cfs-coffee: Fix build warning

Fix this build warning, which is generated if micro logs are enabled:

In file included from ../../core/cfs/cfs-coffee.c:59:0:
../../core/cfs/cfs-coffee.c: In function 'read_log_page':
../../cpu/cc2538/./cfs-coffee-arch.h:145:24: warning: passing argument 1 of 'cfs_coffee_arch_read' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
   cfs_coffee_arch_read((buf), (size), (offset))
                        ^
../../core/cfs/cfs-coffee.c:757:3: note: in expansion of macro 'COFFEE_READ'
   COFFEE_READ(lp->buf, lp->size, base);
   ^
../../cpu/cc2538/./cfs-coffee-arch.h:176:6: note: expected 'void *' but argument is of type 'const char *'
 void cfs_coffee_arch_read(void *buf, unsigned int size, cfs_offset_t offset);
      ^

COFFEE_READ() needs to write to lp->buf, so the target type of this
pointer cannot be qualified as "const".

Signed-off-by: Benoît Thébaudeau <b3efeb72fa9d3102885911404c5fbece843f6c8f@gmail.com>

arurke/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,arurke/contiki