fix broken links

building.md

@@ -5,18 +5,19 @@
 For feedback, suggestions, etc. - please add a post to the [Vogons thread][vogons-thread].
 
 ## BOM
-There's a BOM file [here](kicad/fabrication/ITX-Llama-v1revE-BOM.xlsx), ready to be used with JLCPCB's assembly service.
+There's a BOM file [here][bom], ready to be used with JLCPCB's assembly service.
 In addition to the components that can be sourced through JLC/LCSC, you need to order a few extras elsewhere:
-- Vortex86EX SOM - [DMP](som)
-- Crystal CS4237B - Search for "cs4237b" on [eBay](ebay)
-- ATTINY84A-PU - [DigiKey](digikey), [Mouser](mouser), etc
+- Vortex86EX SOM - [DMP][som]
+- Crystal CS4237B - Search for "cs4237b" on [eBay][ebay]
+- ATTINY84A-PU - [DigiKey][digikey], [Mouser][mouser], etc
 
 For programming the ATTiny, you need a hardware AVR programmer. There are plenty out there.
 
 If you want emulated modem:
-- D1 Mini v4.0.0 (ESP8266) - [AliExpress](d1mini)
+- D1 Mini v4.0.0 (ESP8266) - [AliExpress][d1mini]
 
 [vogons-thread]: https://www.vogons.org/viewtopic.php?t=93480
+[bom]: kicad/fabrication/ITX-Llama-v1revE-BOM.xlsx
 [som]: https://shop.dmp.com.tw/INT/products/25
 [ebay]: https://ebay.com
 [digikey]: https://www.digikey.com/en/products/detail/microchip-technology/ATTINY84A-PU/2774082