fbt: support for LOADER_AUTOSTART; post-build size stats (#1594)

* fbt: restored LOADER_AUTOSTART support
* scripts: added fwsize.py wrapper for size command; fbt: changed size post-build stats to fwsize.py call
* fbt: removed size wrapper
* fbt: added stats for binary flash size in pages
* fbt: hint on build options details
* scripts: fixed fwsize.py for *nix

fbt_options.py

@@ -1,5 +1,7 @@
 import posixpath
 
+# For more details on these options, run 'fbt -h'
+
 
 # Default hardware target
 TARGET_HW = 7
@@ -59,6 +61,9 @@ SVD_FILE = "debug/STM32WB55_CM4.svd"
 # Look for blackmagic probe on serial ports and local network
 BLACKMAGIC = "auto"
 
+# Application to start on boot
+LOADER_AUTOSTART = ""
+
 FIRMWARE_APPS = {
     "default": [
         "crypto_start",