Add information about booting to SafeStrap recovery

In order to make it easier to eventually install LineageOS
to the stock partition, we need Android recovery working.

Let's add some notes about booting to SafeStrap from
droid4-kexecboot.

Signed-off-by: Tony Lindgren <tony@atomide.com>

README

@@ -76,6 +76,13 @@ CMDLINE=console=tty0 console=ttyS2,115200 fbcon=rotate:1 debug earlycon ro \
 rootwait rootfstype=ext4 root=/dev/mmcblk0p1
 TIMEOUT=3
 
+LABEL=SafeStrap recovery
+PRIORITY=1
+DTB=/boot/safestrap/kexec/devtree
+KERNEL=/boot/safestrap/kexec/kernel
+INITRD=/boot/safestrap/ramdisk-recovery.img
+CMDLINE="androidboot.safestrap=recovery"
+
 Note that there is currently nothing updating the kernels extracted from
 rom slots if those kernels get updated and you have to update the kernels
 manually.
@@ -91,6 +98,19 @@ information on boot.cfg file:
 
 https://github.com/kexecboot/kexecboot/blob/master/res/boot.cfg
 
+To kexec boot to SafeStrap from droid4-kexecboot, you can just extract
+the files from Safestrap-maserati-v3.75.apk into your boot directory
+and use the above boot.cfg configuration for "SafeStrap recovery". To
+extract the Safestrap-maserati-v3.75.apk files you can do:
+
+$ mkdir -p boot/safestrap
+$ cd boot/safestrap
+$ mkdir tmp
+$ unzip -d tmp /tmp/Safestrap-maserati-v3.75.apk
+$ unzip -d tmp tmp/assets/install-files.zip
+$ mv tmp/install-files/etc/safestrap/* .
+$ rm -rf tmp
+
 
 BUILDING FROM SOURCES