I could not find the word "boilt" in any of the standard English dictionaries. A further research led me to a fact; there was a great deal of spelling variation in the early centuries of its use. Regional dictionaries show examples of word "boilt", especially in Scotland, Ulster, the Isle of Man, and parts of the USA (especially those influenced by Scots-Irish). A Scots poetic example from 1790: 'Twa pints o' weel-boilt solid sowins' [an oat-meal beverage].
With verbs which have two -ed forms, such as spoiled and spoilt, the situation is interesting and not entirely understood. The -t ending is rare in American English, certainly. In British English, an aspectual distinction is usually involved. The -ed form is used when the duration of an action or the process of acting is being emphasized, and the -t form when something happens once, or takes up very little time, or the focus is on the result of a process rather than on the process itself. Consider "spoiled" and "spoilt".
Credits: David Crystal (original research and most text, credibility verified.)