Standardise bootloader articles to use "U-Boot"

[greenbreakfast]

Task: Standardise wording to “U-Boot”

Scope

• All .md and .mdx files in docs/bootloader/

Required edits

1. Phrase updates

   • bootloader command line → U-Boot prompt
   • Stand-alone bootloader → U-Boot
     Exception: keep bootloader if the sentence is explicitly contrasting different bootloaders (e.g. legacy bootloader).

2. Capitalisation & style

   • Use “U-Boot” (capital U, hyphen, capital B).
   • Keep lowercase u-boot only in filenames, URLs, commands, or code blocks.

3. First reference in each article

   • Introduce as “U-Boot bootloader (hereafter U-Boot)”; afterwards use just U-Boot.

4. Headings & sidebar titles

   • Rename to include U-Boot instead of bootloader where applicable.

5. Snippets & links

   • Update link texts and labels to match the new terminology.
   • Do not rename snippet/component filenames (code references stay lowercase).