Elbert: convert eMMC to ext4 and remove PFR (#100)

Summary:
We have seen file stability issues with the eMMC using btrfs: blocks are corrupted after power cycling the BMC. Converting to ext4 resolves the issue. Also removes PFR tools from the elbert image since they are no longer needed.

### Testing
On Elbert, verified that the ext4 file system is mounted:
```
root@bmc-oob:~# dmesg | grep -i mmc
[   29.140826] mmc0: SDHCI controller on 1e750100.sdhci [1e750100.sdhci] using PIO
[   30.099606] mmc0: new HS200 MMC card at address 0001
[   30.159849] mmcblk0: mmc0:0001 008G70 7.28 GiB
[   30.230762] mmcblk0boot0: mmc0:0001 008G70 partition 1 4.00 MiB
[   30.305778] mmcblk0boot1: mmc0:0001 008G70 partition 2 4.00 MiB
[   30.376983] mmcblk0rpmb: mmc0:0001 008G70 partition 3 4.00 MiB, chardev (247:0)
[   33.464624] EXT4-fs (mmcblk0): mounted filesystem with ordered data mode. Opts: (null)
```
Also stressed power cycling the BMC after writing files and observed no block errors.

Pull Request resolved: facebookexternal/openbmc.arista#100

Reviewed By: tao-ren

fbshipit-source-id: 9848b5296e

meta-facebook/meta-elbert/conf/machine/elbert.conf

@@ -23,8 +23,7 @@ PREFERRED_VERSION_rest-api = "0.1"
 PREFERRED_VERSION_fscd = "0.3"
 
 MACHINE_FEATURES += "tpm2"
-MACHINE_FEATURES += "emmc"
-MACHINE_FEATURES += "pfr"
+MACHINE_FEATURES += "emmc emmc-ext4"
 
 # kernel-module-cpld
 MACHINE_ESSENTIAL_EXTRA_RDEPENDS += "           \