nano: needs mmu

Fix below build error if !BR2_USE_MMU:

text.o: In function `execute_command':
text.c:(.text+0x7f8): undefined reference to `fork'
text.o: In function `do_int_speller':
text.c:(.text+0x3480): undefined reference to `fork'
text.c:(.text+0x34f4): undefined reference to `fork'
text.c:(.text+0x3568): undefined reference to `fork'
text.o: In function `do_alt_speller':
text.c:(.text+0x3914): undefined reference to `fork'
collect2: ld returned 1 exit status
make[3]: *** [nano] Error 1

[Peter: fork() only in full version, so force tiny for !mmu]
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>