(MMP) Make the battery percentage reading work

[Sichroteph]

• Do a correct implementation of the .batt-perc.png update
• Fix Batmon
• Fix all the gpio59 calls
• Read axp directly
• Generate .batt-perc.png even if device is charging

[Sichroteph]

Commit :
63b28fba4dca62c8de48e75e29a8e45473db1c62

[schmurtzm]

Could be improved now thanks to direct access to axp registers.
Does it worth the effort ?
I keep it "in progress" for now.