I'd try a few iterations of tests to verify that things are working and connected properly. Boot the BBB off of it's eMMC if that still has the standard Angstrom boot on it; or have you reflashed the eMMC to boot your LFS? My thinking is that you're booting off of the uSD card to run LFS and the default Angstrom is still on the eMMC. So that's why i recommend you boot off of the default. You should be able to ssh to the BBB; yes the default boot performs DHCP; however you can connect to the network over USB and the address is 192.168.7.2. The relevant information about this is also in Beagle.org's getting started
guide. This therefore verifies that there are no issues performing ssh from your macbook. Once attached you can alter the network configuration to be static if this is your preference and then ssh to both the 192.168.7.2 USBNet address as well as the Ethernet static IP; or likewise discern any DHCP address when you ssh over the USBNet and then you'll know what address to reach it to via eth0.
By the way, if all else fails; if you have a Linux bootable workstation; try and perform ssh from that to your BBB. For all things, I consider the Linux variation to be the most stable and sometimes whatever you have installed on your PC may be some free ware substitution of ssh and other Unix/Linux like utilities and they may not work, or may not work well.
From this you'll validate that your workstation can successfully ssh to the BBB, connections are valid, and so forth.
Next, botting LFS, if you can use the standard SSH, then try that; I'm not highly versed in whether dropbear is the standard, but on my systems I see /usr/bin/sshD for an SSH daemon; and that sounds pretty standard to me.
Note that the BBB has some reasonable space on the eMMC and if you boot off of a uSD, then that can be fairly large; I use 8G uSD cards myself, therefore between kernel and RFS I have about 6.5G of leftover free space; if not more; and that's without me minimizing my distribution in any manner. My thinking there is don't fret over using SSHD over dropbear as far as size goes; they may also be comparable in size too.
If you can get it working readily with standard sshD, then it becomes solely a dropbear question. Sorry, little experience there except to say that in the typical SSH scenario; if you follow the steps I've suggested, your workstation will have saved keys in it's known_hosts file for the addresses of your BBB and the next boot if you're using the same IP address or are assigned that same IP address via DHCP, this may cause the client to have difficulties because it remembers different keys for a different system, but same IP. As a result sometimes on the client you have to remove the .ssh/known_hosts file. Perhaps there is a similar dropbear equivalent. From what you wrote you generated that. SSH does that by asking if you wish to add trust for that peer via yes/no question and once you say yes, it adds that host information to the known_hosts file.