bless: add page (#4439)

pages/osx/bless.md

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+# bless
+
+> Set volume bootability and startup disk options.
+
+- FOLDER MODE: bless a volume with only Mac OS X or Darwin, and create the BootX and boot.efi files as needed:
+
+`bless --folder {{/Volumes/Mac OS X/System/Library/CoreServices}} --bootinfo --bootefi`
+
+- MOUNT MODE: set a volume containing either Mac OS 9 and Mac OS X to be the active volume:
+
+`bless --mount {{/Volumes/Mac OS}} --setBoot`
+
+- NETBOOT MODE: set the system to NetBoot and broadcast for an available server:
+
+`bless --netboot --server {{bsdp://255.255.255.255}}`
+
+- INFO MODE: gather information about the currently selected volume (as determined by the firmware), suitable for piping to a program capable of parsing Property Lists:
+
+`bless --info --plist`